Propeller advice mercruiser

MCL

Petty Officer 2nd Class
Joined
Feb 13, 2010
Messages
109
Hello

I have a 20ft cuddy cabin boat. The engine is a 2.5l ford turbodiesel that ive built myself and mated to a alpha one drive with a 1.5 gear ratio.I dont know the output but would quess somehwere in the 130hp range. I built this engine to economically cruise,its not meant to run wot and im not that interested in it anyway. Fuel consumption is very low,I have not made a exact measurement but somewhere in the 13-14l/h region cruising at 27-28mph. I have tried a few different propellers and still not 100% happy and looking for suggestions.

Ive tried a solas hr titan 4 in both 17 and 19 pitch.The 19 pitch would barely plane but when it did the cruise was excellent, I dropped down to a 17 pitch. This prop was quite good but had some downsides,it would cruise at 25mph at 2800rpm, holeshot was quite good and could pull a wakeboarder easily with a few people onboard. The quality of the propeller was quite poor,it was out of balance and even the hub was out of round(brand new prop),my overall impression was that it was quite a heavy prop that requires alot of power to spin it.It would max out at 33mph and rougly 3100-3200rpm. To little wot rpm meant i was labouring the engine and going down in pitch would cruise speed even more. It had alot of stern lift and the boat would porpoise quite easily when trimmed. One problem with the 4 blade titan was that it would not slip, i had problems getting the engine up in rpm to start building boost and also power.

Next and current prop is a mercury vengeance 19 pitch, quality of this prop is very good. Compared to the titan the vengeance slips alot during planing which makes the engine get up to the "powerband" alot faster,still planing speed is rougly the same.During planing the nose of the boat goes up towards the sky which it never did with the titan.At 2800rpm im cruising at 26mph and at 3000 at 29mph. Weirdly enough although this prop doesnt have cup or rake it allows me to trim the nose up alot more. It tends to slip alot and this is confirmed using the slip calculator(18%).If I wot it the rpms jump straight to the governor max speed of 4000rpm, speedwise I saw 36mph but it would go more if I would keep it at wot. Also at cruise the aft tends to sit quite deep in the water compared to the titan.

Im quite happy with the current vengeance,ideally I would like a prop to get just a bit more speed out of the same rpm and maybe get the slip numbers a little bit down. Any suggestions on pitch or propellers?

Fyi I would not recommend the solas propellers, at least not for the big boats, the quality is really subpar when compared to the mercury props. I do have a solas saturn on a 15hp suzuki that is really flying after some modifications and refinishing, so maybe for the smaller boats they are ok.
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
40,582
20ft Cuddy with 130hp motor

4 blade
17P 2800 rpm 25 mph = 17%
17P 3200 rpm 33 mph = 4% (somethings wrong)

Vengeance 14.5 diameter
19P 2800 rpm 26 mph = 13.5%
19P 3000 rpm 29 mph = 10%
19P 4000 rpm 36 mpr = 16% (ventilating)

Assuming the ventilation plate is at or just above the bottom of the hull

The vengeance allows for more trim because there is less lift like with the 4 blade. The 10% slip with it seems quite good for the motor. You could add a slight but of cop to the prop to reduce the rpm and probably decrease slip, but this can also takes more power. I wouldn't recommend adding cup because this prop is allowing the motor to reach close to its WOT rpm

Running a motor with less load and higher rpm does not increase fuel consumption, and you mentioned at the first that top speed is not the goal. Everyone seems to want to have the cruising rpm at or below 2500, but this may not be achievable along with long life.

Suggest trying a 15.25 diameter 17P. Find a prop shop that will let you try some
 

WesNewell

Chief Petty Officer
Joined
Jan 3, 2018
Messages
497
Don't know why you think something is wrong with your 17P slippage. it isn't. At 2800rpm you aren't on full plane yet, so there's more drag making for more slippage. At faster speeds there's less drag because the boat on full plane with less slippage. If your rpm readings and speeds are correct, then the slippage numbers are very close. Actually the calc I used show only 3% slippage at 3200rpm 33 mph, which is excellent.
 

MCL

Petty Officer 2nd Class
Joined
Feb 13, 2010
Messages
109
Hello,thank you for your answers.I got to admit the numbers for the 17 pitch titan might be a bit off because im working out of memory.I remember doing the slippage calculations back then and got around 8-9% slippage. I agree that the boat is not fully planing at 25mph, its almost there.

The numbers for the vengeance are accurate because i took pictures from my rpm gauge and plotter while running.
I get 22% slippage with the vengeance at 2800rpm,exact speed numbers are 22,6 knots(26,0mph)
Slippage is 18% slippage at 3000rpm with the speed being 25,5 knots(29,3mph)
I dont know how you get the low slippage numbers?The high slippage for the wot run is because i did not allow the boat the pickup the speed since i dont like run the rpm that high,it was still picking up speed when i let off.

There is no dyno numbers. The engine is custom built by myself.The block is a ford 2.5 turbodiesel out of a transit van(90-00).Cast iron block and head, 2valves per cylinder with pushrods, bosch mechanical vp rotary fuel pump. In car form this engine had 115hp(3500rpm) with a intercooler running 0,5 bars of boost.
Im running same boost numbers but with a bigger turbo,intercooled and uprated 130hp injectors,exhaust is going out thru hull with a muffler since i dont like the idea of burning up the drive incase of waterpump failure.
Incase someone is interested in diy marinizing of car engines all i can tell you that it is a massive undertaking.

To get back to the point planing with the titan felt like you had massive wheels and the traction never breaks, while with the vengeance when you throttle it feels like your spinning the tyres,rpms jump up and the speed picks up gradually. The reason why i want to drop the rpms a tad is just because this engine starts to get real noisy after 3000rpm. While the injection pump governor is set at max speed of 4000rpm there is no way you would want to run the engine at that speed.I think if you look at volvo penta diesels most of them have the max rpm at 3600rpm or so(at least the older models). This engine is not a rever by any means,if i remember correctly they ahd max torque at 2400rpm and 90% of max torque starting from 1800rpm.

So ideally would like just to see 100-150rpm less for the same speed and the wot numbers in the 3600rpm range.It feels a bit underpropped since if i cruise at 3000rpm and give it full throttle the rpms will pretty much jump straight to 4000.
Regarding ventilation plate height im not sure, i redid the transom since i converted from volvo the merc and remember setting the x dimension maybe 1/2-1 inch above merc recommended x dimension. I would need to fit a gopro or such to film the outdrive while underway but i would say that im not running the drive overly high out of the water.
 

ahicks

Captain
Joined
Sep 16, 2013
Messages
3,957
Rather than trying to find the perfect off the shelf prop, I would suggest you take the prop that works the best, and have a good prop shop set it up for you. They have a lot of variables they can play with (cupping for instance) to put a big smile on your face every time you use it.

You have a custom engine,and have obviously taking a lot of time to get those specifications just right to get to the point you're at. Use that same slow going get the details right process to get this prop right. Keep in mind you're trying to do everything a gas engine will do, only the width of your power band is much narrower. Surprised to hear you're doing as well as you are. Good job!
 
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