Speed - Power - RPM - Pitch Question

[David Bourgeois]

Borg Warner has a ratio of 2.57:1, so you are close at 2.6. useing my prop calculator with a Perkins 6-354 and a 2.57, you should get 8.7 knots if your boat wieghs around 15 ton. you would have to swing a 24x16.6 4 blade or a 25.7 x 17 3 blade prop these props should put you close if your engine will turn up 2600 rpm. If the boat is heavier than 15 ton, then the speed will be slower accordinly. I would have to double check, but the Perkins has a rated rpm of 2500 or 2600 rpm, and it should turn this under full loaded condition, and should turn a bit more under a light ship condition, and should be propped accordinly. It is better to have a prop that is a little light, than a prop that is a little heavy so that you dont overload the engine and shorten its life.

Good luck , David Bourgeois

[Big Pete]

Hi Ian M,

Can we have some feedback on what you found?

We are all in suspense.

BP

[IanM]

The cleaning was for the most part successful. The bottom paint was in excellent condition and the zincs still had some life left. We'll do a proper haulout in the spring to repaint. Made a disturbing discovery with the prop though, as you can see in the picture. All 3 blades are bent and torn - the one closest in the picture is the worst. I suppose this would explain the lack of speed. The surprising part is that there was no real noise or vibration. The boat won't be getting much use between now and the spring, so we'll leave it this way for now and either repair or replace the prop in the spring. The current prop is 24x24. We do have a spare prop, 26x can't remember the pitch. The blades seem to be narrower though.

The engineer in me tends to agree that when the service conditions change, the propulsion system needs to be reevaluated. Re-pitching to take advantage of the lighter load makes a lot of sense on paper, but given the cost, time, and uncertainty involved I can understand why most of the fishboats that are converted to pleasure use never undergo any propulsion changes.

On an unrelated note, there was a lot of white coral growth on the prop, more than I'd ever seen before. It was growing in a big ball right around the hub. Can anyone recommend a prop coating? I've heard of a clear coat called prop-speed, but never had any experience with it.

I'll post an update when we reach the next step. Thanks for all the help thus far.

[JK]

IanM I reduced your photo size from about 24x18" to something that most PCs will be able to view in a reasonable period of time.
Maybe you should discuss radical propeller designs with JAACosta
Amazing you had no vibrations!

[IanM]

Thanks, I was in a rush posting from work and was planning on resizing when I got home.

[Big Pete]

Hi Ian,

Great picture.

Looks like you will have to fit a new prop!!!!

I suspect the boat was laid up for a while and the coral grew all over the prop, then was worn off when the engine was run, (highest velocity relative to the water at the blade tips).

As you have a spare prop the obvious thing is to try that and see how it goes, no point in buying a new prop for a small improvement in performance.

If the blade area of your spare prop is less than the present one you may suffer more slip, but if your "deadweight" is reduced that may cancel out the effect.
At least you will be able to sell the old prop for cash, unless you can part exchange it!

BP :