New Motor, New Prop

[TOM]

We have just repowered with a new Yanmar 4JH4-TE 75HP . It has not been a straightforward repower {I will write about the problems we encountered and total overall cost on our website www.byamee.com }
Our latest problem is that we need a new prop . Our setup is that the new motor has a 2.15:1 reduction which then goes into a V-drive that has a 4:3 reduction {1.34:1} the end result been a n overall reduction of 2.86:1 .With our displacement at 12 tons and 36 foot waterline length this necessitates a 22 inch prop ! I don’t want a 22 inch prop and even if I did I could not put it on as the end of the shaft has been cut down to 1 and a quarter inch to fit the old kiwi prop. The biggest prop I can take is about 19 inch.

Our solution has been to swap over the V-drive gears making it a 3:4 step up giving an overall reduction of 1.6:1 {still with me …good} .The prop calc which has come back now says 16 inch with 10 inch pitch . I know the prop calc is right but I cant help been concerned that it is on the smallish side and has not a lot of pitch.

Can you guys please tell me is my shaft overall going to spin faster with this lower reduction ,will this wear out the cutless bearing quicker ? .Will my fuel consumption be better with a smaller prop . Will the wear on our motor be greater. We have spent all this money and want to get it right .

As always looking forward to your responces

www.byamee.com

[Boracay]

When I put in the new engine I kept the original prop as I figured the engine was sort of similar to what I had.

What I'm thinking is why not try with the prop you have now. Take the boat out and (keeping running in recommendations in mind), do some measured runs at various rpm's checking with the GPS. Make careful notes allowing for tide, wind and current. Even see how you go against some serious chop and swell.

From that you should be able to go back to the prop people (or this Forum!!!) and get some informed opinions on what is best.

If money were no object I'd suggest a feathering prop (see Forum for opinions).

I found that my original prop had way too much pitch. Even with the prop shop taking off more than I requested I still have a bit too much (2300 rpm as opposed to a desired 2500). But I think that I can live with what I have. The boats not perfect anyway...

Don't forget that as fouling builds up it will chew up some of your speed and power. My experience suggests that a slightly smaller, under pitched prop may give a better long term result, as long as you take care not to over rev the engine.

Your engine has lots of power so the old rule of running at 80% power may not be applicable. My experience suggests a possibility of using 20 - 40% power for long periods.

[Chief Engineer]

Pardon me if I am out of line here, but did you do your calculations before you got the engine?

What was your reduction with the old engine?

[TOM]

reduction on old engine was 1.52:1 then a further reduction of 1.34:1 on the Vdrive giving an overall reduction of 2.04:1. The calculations were way beyond my capability and were done by Philip Haynes the guy who makes the v-drives so i imagine he knows what he is talking about,if not iam screwed!.

Boracay I forgot to mention that the old kiwi prop is left hand and my new setup requires a right hand prop .Know anyone who wants to buy a good secound hand Kiwi prop ! . Thanks for your other advise i ordered the new prop 16/10 from Austral props on friday should be here tuesday ,my main concern is that i will not have enough torque ,my wife is a big fan of torque as we have had a few unplesant exsperiences doing 1.5 knots into a 30 knot wind under power.

And lets face it keeping the girl happy means i can keep on cruising.

P.S Have you got the sails up yet ! Remember it is a sailing boat lol

[Boracay]

Hi Tom,
'Looks like you've got it all under control.
Got to keep the Admiral happy at all costs.
Not sailing but had lazy jacks put in on Friday so getting there.
The TV is more important...

[ColdFusion]

I can't answer your questions but perhaps this might help?

Propeller Calculator / Prop Calculator

Or if you really want a headache, this provides more parameters, including calculated prop RPMs:

http://www.alberg30.org/maintenance/...r/propcalc.xls

[SkiprJohn]

I think this is a subject we can all agonize over. Many of the older Mariners and Fujis came with a Perkins 4-107 which was rated at about 45 hp. They had 1:1 Velvet drive transmissions and 12" props. I have an engine with about the same hp rating, a 2:86 Velvet Drive and use a 18/10 prop. Comparatively speaking I think your 16/10 prop will be fine but honestly you won't know until you get it in the water and do some tests. You can have good quality props repitched without a haulout, just have a diver remove it. It should be easy if it hasn't been in the water for years.
regards

[SkiprJohn]

Yes, your prop will spin faster and your cutless bearings will wear more because of it but it won't be a significant difference when you replace them in 10 years or so.
regards,