Discussion
Truth is there are various calculations that can be made utilising hull form, weight, max rpm, gearbox ratio, HP, theoretical slip and length. But they only really give you a starting point and you already have that with your existing propellor.
Bear in mind propellors are about compromises. Consider it as taking a journey in your car and only being able to select a single gear for the whole journey. Were the journey to be mainly motorway you would select the highest possible gear that you could still achieve pull off with and sacrifice acceleration. Were you just running around town you may select second gear and sacrifice the fact that you can't get past 50. (Cerberas excepted
).
So first question is how do you rate your requirements for acceleration versus outright speed.
Then we look at type of propellor.
If we want outright speed then we may lift the engine and send the propellor into semi surface piercing territory and utilise a cleaver or spoon variety.
If on the other hand we want a tremendous pulling power then we drop the engine deeper and utilise a large blade area and lower pitch.
If we want the bow to rise under power then we go for a higher rake to the blades and may well set the engine back on a bracket to help leverage. Conversely if the bow rises too far or we need to lift the stern we do the opposite amongst other things.
Once we know what we are trying to achieve we also need to pick a blade cupping that works well for the situation improving bite and decreasing slip. Cuppong can add the equivalent of 2 or 3" pitch making just quoting pitch figures useless.
Material selection makes a huge difference with a typical say 19" pitch aluminium prop being replaced by a 21" Stainless prop with no loss of top end RPM.
Other than that it's quite a straight forward process.
An alternative to the above is to find a witch doctor / clairvoyant.
Tell us the following though and I'll try to help.
Existing prop used, Type and dia / pitch.
Age / details of motor.
Top end RPM achieved.
GPS speed achieved.
Typical loading on boat (No of persons etc).
Depth of propshaft below hull.
What you want to achieve i.e. pull 14 skiers at a time or just achieve outright race speeds.
Bear in mind propellors are about compromises. Consider it as taking a journey in your car and only being able to select a single gear for the whole journey. Were the journey to be mainly motorway you would select the highest possible gear that you could still achieve pull off with and sacrifice acceleration. Were you just running around town you may select second gear and sacrifice the fact that you can't get past 50. (Cerberas excepted
).So first question is how do you rate your requirements for acceleration versus outright speed.
Then we look at type of propellor.
If we want outright speed then we may lift the engine and send the propellor into semi surface piercing territory and utilise a cleaver or spoon variety.
If on the other hand we want a tremendous pulling power then we drop the engine deeper and utilise a large blade area and lower pitch.
If we want the bow to rise under power then we go for a higher rake to the blades and may well set the engine back on a bracket to help leverage. Conversely if the bow rises too far or we need to lift the stern we do the opposite amongst other things.
Once we know what we are trying to achieve we also need to pick a blade cupping that works well for the situation improving bite and decreasing slip. Cuppong can add the equivalent of 2 or 3" pitch making just quoting pitch figures useless.
Material selection makes a huge difference with a typical say 19" pitch aluminium prop being replaced by a 21" Stainless prop with no loss of top end RPM.
Other than that it's quite a straight forward process.
An alternative to the above is to find a witch doctor / clairvoyant.
Tell us the following though and I'll try to help.
Existing prop used, Type and dia / pitch.
Age / details of motor.
Top end RPM achieved.
GPS speed achieved.
Typical loading on boat (No of persons etc).
Depth of propshaft below hull.
What you want to achieve i.e. pull 14 skiers at a time or just achieve outright race speeds.
MOTORVATOR said:
Tell us the following though and I'll try to help.
Existing prop used, Type and dia / pitch. 13 pitch, diameter not sure off hand
Age / details of motor.mariner 40bhp, again age unknown, but not new
Top end RPM achieved. no way of knowing this as no rpm dial?
GPS speed achieved. on average 29mph
Typical loading on boat (No of persons etc).around 30stone, 2 adults and a child
Depth of propshaft below hull.id guess 10"[/b]
What you want to achieve i.e. pull 14 skiers at a time or just achieve outright race speeds
loking for more top end, she pulls ok, gets out of the hole fairly well (doel fins would help if changing pitch changed this) its just on calm water id like a bit more top end.
ive tried to answer your questions the best i can mate without having the boat in front of me, thank you so much for the help. im lost in all this i have to sayExisting prop used, Type and dia / pitch. 13 pitch, diameter not sure off hand
Age / details of motor.mariner 40bhp, again age unknown, but not new
Top end RPM achieved. no way of knowing this as no rpm dial?
GPS speed achieved. on average 29mph
Typical loading on boat (No of persons etc).around 30stone, 2 adults and a child
Depth of propshaft below hull.id guess 10"[/b]
What you want to achieve i.e. pull 14 skiers at a time or just achieve outright race speeds
loking for more top end, she pulls ok, gets out of the hole fairly well (doel fins would help if changing pitch changed this) its just on calm water id like a bit more top end.
Edited by T_Pot on Sunday 8th May 21:23
T_Pot said:
MOTORVATOR said:
Tell us the following though and I'll try to help.
Existing prop used, Type and dia / pitch. 13 pitch, diameter not sure off hand
Age / details of motor.mariner 40bhp, again age unknown, but not new
Top end RPM achieved. no way of knowing this as no rpm dial?
GPS speed achieved. on average 29mph
Typical loading on boat (No of persons etc).around 30stone, 2 adults and a child
Depth of propshaft below hull.id guess 10"[/b]
What you want to achieve i.e. pull 14 skiers at a time or just achieve outright race speeds
loking for more top end, she pulls ok, gets out of the hole fairly well (doel fins would help if changing pitch changed this) its just on calm water id like a bit more top end.
ive tried to answer your questions the best i can mate without having the boat in front of me, thank you so much for the help. im lost in all this i have to sayExisting prop used, Type and dia / pitch. 13 pitch, diameter not sure off hand
Age / details of motor.mariner 40bhp, again age unknown, but not new
Top end RPM achieved. no way of knowing this as no rpm dial?
GPS speed achieved. on average 29mph
Typical loading on boat (No of persons etc).around 30stone, 2 adults and a child
Depth of propshaft below hull.id guess 10"[/b]
What you want to achieve i.e. pull 14 skiers at a time or just achieve outright race speeds
loking for more top end, she pulls ok, gets out of the hole fairly well (doel fins would help if changing pitch changed this) its just on calm water id like a bit more top end.
Edited by T_Pot on Sunday 8th May 21:23
A big caveat there though as without a rev counter we don't know if you are achieving full rpm and hence power.
Assuming the Mariner is old we are talking 2 stroke and it's important as the torque curves are vastly different.
You could lift the engine by a couple of holes until you get ventilation happening on pull out Then drop back to just stop it. What this does is decrease drag of the engine bullet / leg in the water. But we also need to keep an eye on temperatures as it is possible to come up too high and starve the engine of cooling water.
Do you have power trim on the engine? If so are you using it? Trim out once you are on the plane then it reduces the wetted area of hull and increases speed. An alternative if you haven't is to move the thrust pin out a notch but it may make you look a little skywards on pullout.
I'm not a lover of doel fins particularly if you are looking to increase speed. If you consider it not one manufacturer offers a bigger cavitation plate as a solution to powering a boat so why do you think they don't take the option up?
Then be realistic about what you can possibly achieve. Theoretically we could possibly get maybe 50 mph out of this set up but it would be totally unuseable for anything other than leaving everybody and their seats behind and just getting vmax after a difficult time getting it up on the plane.
Check the RPMs first to make sure you are making full revs which I believe to be around 6k on this engine but need to check. Assuming you are and I'm right about the ally prop bit then I would suggest looking for something like a 15 pitch stainless but unfortunately you are in too low a performance band to allow use of some of the more exotic props like high fives that would retain acceleration whilst getting top speed.
But RPM first, if you are not making full revs now adding pitch will just slow you further.
your spot on mate, its a 2 stroke with an ally prop blade yes.
from the lowest ppoint of the hull at the rear, to the blades is about 12" from looking at pics now, maybe an inch more but thats it.
there is no power trim, she is set currently to the highest but one pin on the trim plate though. this seems to give a good ride height, and the front isnt trying to point north at any time lol
how do i find the rpm, the engine has a 12v cable output for recharging battery if that makes any odds?
50mph would be painful on this boat, but if i could 35 to 40 id be very very happy.
regards the doel fins, i take your point on board, and agree now you say it that no one offers them as standard, so il stay away from them.
thanks again, very helpful
from the lowest ppoint of the hull at the rear, to the blades is about 12" from looking at pics now, maybe an inch more but thats it.
there is no power trim, she is set currently to the highest but one pin on the trim plate though. this seems to give a good ride height, and the front isnt trying to point north at any time lol
how do i find the rpm, the engine has a 12v cable output for recharging battery if that makes any odds?
50mph would be painful on this boat, but if i could 35 to 40 id be very very happy.
regards the doel fins, i take your point on board, and agree now you say it that no one offers them as standard, so il stay away from them.
thanks again, very helpful
T_Pot said:
your spot on mate, its a 2 stroke with an ally prop blade yes.
from the lowest ppoint of the hull at the rear, to the blades is about 12" from looking at pics now, maybe an inch more but thats it.
there is no power trim, she is set currently to the highest but one pin on the trim plate though. this seems to give a good ride height, and the front isnt trying to point north at any time lol
how do i find the rpm, the engine has a 12v cable output for recharging battery if that makes any odds?
50mph would be painful on this boat, but if i could 35 to 40 id be very very happy.
regards the doel fins, i take your point on board, and agree now you say it that no one offers them as standard, so il stay away from them.
thanks again, very helpful
Ok ally prop is an instant win if you go to stainless. Due to strength they are thinner but also they flex less and therefore are enormously more efficient. This will be the biggest win anyone ever makes with props. Expect to pick up a couple of inch pitch with no noticeable effect on revs but an obvious gain in performance.from the lowest ppoint of the hull at the rear, to the blades is about 12" from looking at pics now, maybe an inch more but thats it.
there is no power trim, she is set currently to the highest but one pin on the trim plate though. this seems to give a good ride height, and the front isnt trying to point north at any time lol
how do i find the rpm, the engine has a 12v cable output for recharging battery if that makes any odds?
50mph would be painful on this boat, but if i could 35 to 40 id be very very happy.
regards the doel fins, i take your point on board, and agree now you say it that no one offers them as standard, so il stay away from them.
thanks again, very helpful
Rev counter I'm not totally sure how you wire in without a little more info. The 40 mariner could be a yamaha or mercury base engine and we need serial numbers to find out. Either way we should be able to just pick up a couple of wires off the loom but let's check. Alternatively just borrow someones digital tach and point it at the flywheel.
I think I might have confused you with the shaft height. If you look at the prop above that there is a plate. This is called the cavitation plate and wants to be approx 1" below the hull or even level with. Any more and you are needlessly dragging stuff through the water.
I'm surprised you are out on the last but one notch and makes me think that you may be inadvertently correcting one problem by introducing another.
The most efficient way for a motor to drive is for the propshaft to be inline with direction of travel but sometimes a little sacrifice of bowlift will help to reduce friction on the bottom of the hull. By trimming an engine out though you also reduce the amount of gear in the water and therefore drag but if you trim too far you should get the prop to break the surface of the water.
My guess is the engine is set up a little too low on the boat allowing you to trim out so far without detrimental effect. That's where I would start looking before getting into a selection of props.
Right,
Lets keep it simple and cheap.
All this talk of stainless props, rake, lifting the engine etc etc is all quite correct but not practical to do - especially when we don't even know what RPM we're making.
I would make the following comments:
1. Its an old 2 stroke 40hp without power trim and no tacho.
2. 15' Fletcher. Assume that the engine is at about the right height. (A gain may be made by lifting it but lets get closer with the prop first)
3. Can we find the RPM it's making? OP does it sound like its screaming / rattling or is it sounding a bit slow revving? If it sounds like it's revving high (my guess) try a propeller with a larger pitch and see what happens. Just get a secondhand alloy one from ebay. If it makes no improvement then put it back on ebay.
4. Might be worth buying a tacho for the engine. Google tinytach - this is a self contained unit.
5. So, get closer to ideal with some cheap alloy props and maybe get a tacho.
Lets keep it simple and cheap.
All this talk of stainless props, rake, lifting the engine etc etc is all quite correct but not practical to do - especially when we don't even know what RPM we're making.
I would make the following comments:
1. Its an old 2 stroke 40hp without power trim and no tacho.
2. 15' Fletcher. Assume that the engine is at about the right height. (A gain may be made by lifting it but lets get closer with the prop first)
3. Can we find the RPM it's making? OP does it sound like its screaming / rattling or is it sounding a bit slow revving? If it sounds like it's revving high (my guess) try a propeller with a larger pitch and see what happens. Just get a secondhand alloy one from ebay. If it makes no improvement then put it back on ebay.
4. Might be worth buying a tacho for the engine. Google tinytach - this is a self contained unit.
5. So, get closer to ideal with some cheap alloy props and maybe get a tacho.
I have checked today and the cavetor? plate is more or less in line with the hull, it does look angled up and not flowing inline with the bottom if thats ok?
im going to check the revs before doing anything, just not sure as yet the best way, the digital tacho looks expensive for something i dont need after the initial check.
im going to check the revs before doing anything, just not sure as yet the best way, the digital tacho looks expensive for something i dont need after the initial check.
T_Pot said:
I have checked today and the cavetor? plate is more or less in line with the hull, it does look angled up and not flowing inline with the bottom if thats ok?
im going to check the revs before doing anything, just not sure as yet the best way, the digital tacho looks expensive for something i dont need after the initial check.
Cavitation plate and yes that's fine but bear in mind that angled up bit is using power to raise the bow rather than push the boat along. This is where a higher rake prop will help, you can retain the same amount of bow lift but also drop the shaft to be in line with the direction of travel.im going to check the revs before doing anything, just not sure as yet the best way, the digital tacho looks expensive for something i dont need after the initial check.
For the rev counter you're going to have to give us a little more to go on. There's 21 different models of the 40 listed ranging from 2 to 4 stroke carbed and efi and 2,3 or 4 cylinder and different length legs which is why I wanted you to check where that plate was.
Serial number and I can get a wiring dia and help.
T_Pot said:
where is this number found mate?
i know its a 2 stroke 2 cylinder thats it lol
Ok should be on the side of the saddle (the bit that bolts to the boat and the engine hinges on i know its a 2 stroke 2 cylinder thats it lol
)With the 2 cyl there's a distinct possibility that we are not looking at a 40 HP engine in the true sense as they used to be rated at the flywheel and in more recent years (20 - 25) are rated at the propshaft so taking into account gearbox losses before quoting power output.
Edit Actually a photo of the control would help too just to work out if the harness comes back up the boat.
Well here you go interesting thread.
http://www.marineengine.com/boat-forum/showthread....
1989 yamaha.
Of course doesn't help as I can't find wiring diagrams for that on the merc sites.
Does the loom come back upto a control box with an ignition key at that end?
http://www.marineengine.com/boat-forum/showthread....
1989 yamaha.
Of course doesn't help as I can't find wiring diagrams for that on the merc sites.
Does the loom come back upto a control box with an ignition key at that end?
MOTORVATOR said:
So no loom to the front of boat at all even for engine cut out?
there is an on off switch on the engine itself, next to the choke. there is a 2 wire cable from the engine, which im told is for recharging battery, but as the boat has no battery yet its attached to nothingT_Pot said:
MOTORVATOR said:
So no loom to the front of boat at all even for engine cut out?
there is an on off switch on the engine itself, next to the choke. there is a 2 wire cable from the engine, which im told is for recharging battery, but as the boat has no battery yet its attached to nothingGassing Station | Boats, Planes & Trains | Top of Page | What's New | My Stuff