Prop size

[gaucho]

I just acquired a 1985 Passport 40. It has been repowered with a Beta 50, topsat 2800 rpm. I wouldn't know what the cruising rpm should be. I do know that on the sea trial, we worked the engine wide throttle, and the boat wouldn't do more than 6knots. The engine sounds great, but I really think it's underpropped. A 50 should push that boat much better. But then again Im no expert. I requested a recomendation from MaxProp, and got back a completely different setup from what is currently on the boat. That kind of cemented my doubts, now I am almost convinced Im underpropped. But I could be wrong. What do you guys think?

Passport 40, 40ft, 23000 lbs displacement

Beta 50@2800 rpm, 2:1 transmission, 1.25 shaft

Currently it has a 15x10 fixed 3 blade

Maxprop and the propcalc spreadsheet both gave me a 18x12 recommended prop

Anyone wants to shine some light into this prop dilema? To me it all sounds like black magic.

[Sailmonkey]

We've got a 28 hp kubota engine in a 32' boat spinning a 1.25" shaft and a 16rh10 prop......I'd say you're underpropped.


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[DougR]

You are under propped.
According to my prop program the ideal fixed three blade prop would be a 19.5" X 10.8", and depending on the amount of area available to swing a wheel, you could be using something like a 19x 11.5 x3 or an 18 X 12 X 3 with just a slight bit of efficiency loss because the blade area is not as great.

Anything below 18 inch diameter just doesn't have enough blade area to get the power into the water, and the prop will be cavitating when pushed to higher RPM.

Theoretically, the prop should be large enough prevent the engine from revving to more than its maximum rated RPM, and this means that if the propeller and hull were clean during the sea trial, the engine should have been able to turn more than 2800. Perhaps as much as 3000 or even higher.

Check the amount of tip clearance (distance between the blade tip and hull) that you have with the 15 inch prop and calculate how much diameter you can swing while still maintaining a tip clearance of about 15% of the prop diameter. Go for the largest diameter you can swing and adjust pitch accordingly. ( but not over 20" diameter"

DougR

[a64pilot]

What is the engine rev limited at as in neutral, full throttle?
What RPM do you achieve at full throttle in gear?

[gaucho]

Originally Posted by a64pilot

What is the engine rev limited at as in neutral, full throttle?
What RPM do you achieve at full throttle in gear?

Im going to be back in the boat in a couple of weeks (had to fly back home). I don't know the neutral rpm wide open, but it achieved 2700 wide open in gear during the sea trial. Surveyor suggested a change of pitch to achieve the 2800. Honestly after feeling the sluggishness of the boat at full throttle, Im inclined (and now sure) that the boat is under propped.

My guess (and its just a guess) is that the PO who did the engine repower chose to keep the old prop as to not spend more money, one of those "Ill change it soon", and it just never happened.

How can I know if there is clearance? The boat is in the water, and I can't really take it out once more, unless it's for actually changing the prop ...

I'll ask in the Passport group what prop sizes they have, if an 18 fits another Passport, should fit mine. I guess.

[capt jgw]

One thing to check is the tach accuracy, especially if it's driven off the alternator. Sometimes they'll put a smaller pulley on the alt to get more amps at idle, but that will throw the tach way off. Check it with an optical tach. With a governor speed of 2800, 2700 would be about right. But if that's really 3000, then you're under propped. As mentioned above, compare to what the tach reads WOT in neutral. The actual numbers aren't important, the difference is.

[a64pilot]

If you get an indicated xxxx RPM in neutral, and don't quite get to xxxx RPM in gear, you are not underpropped.
If you easily reach governed RPM in gear, then you are underpropped.
A correctly propped boat it's been my experience will usually get to or close to full governed RPM in reverse, but not quite in fwd.

You measure prop clearance by diving the boat and looking, better learn how or expect to pay someone else to clean it often.


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