Calculating Prop Size

[PGD]

Can anyone offer some advice on how to calculate the prop size I need for my re-engined Fairline Holiday - weight approx 2 - 2.1 t/e (1.85 dry) but new engine weighs more(AD31P 150hp), drive unit is a DP -E. Volvo book says A7 to give 26-30 kts. Any suggestions? Thanks.

 

[adrianm]

Try www.boatdiesel.com, they've got a prop calculation program that I use.

Can't remember whether you need to be a member though.

I expect there are lots of similiar programs dotted around the net.

 

[Guest]

I have no idea if it works this way, but I have a pair of AD31s with A7 props pushing just on 4 tonnes of Targa 29 at 30 kts. A7's (I think) are the longest pitch props you can get on the AD31, so anything shorter would likely overrev the engine.

 

[PGD]

Hi, thanks for that, the book!! says the set up will take A10s and give 40kts+ but leaves out one thing - weigth of boat? Typical of VP! The marina has a spare set of A8s so we'll try these as see if she get onto the plain, if not go for A7s or 6s.

 

[brianrunyard]

Re: Prop calc programs

There's a prop calc program at http://castlemarine.co.uk/pitch.htm

 

[miket]

Re: Prop calc programs

CJR Propellers in Southampton have all the programmes and are extremely helpful.
Think they are on the net.

 

[Scubadoo]

That's interesting my single AD31 DP-E has the A6s and I was comtemplating A5s to improve planing, but didn't like the drop in top speed.

Therefore from my opinion (I could be wrong) that A6 seems a good all rounder - gives me 29/30knots and a reasonable time to plane - however fully loaded with dive gear it can struggle.

Graham were you the one who was concerned about time to plane - prehaps the A7s could be your main problem, ever consider trying A6s.

Of course every boat is different and I guess if PGD tried A8/10 it could stuggle to plane - but have a good top speed (if you can get there!).

RM.

<P ID="edit"><FONT SIZE=-1>Edited by richard_moore on Wed Aug 8 12:33:23 2001 (server time).</FONT></P>

 

[Guest]

Yes, I was that man. But I've fixed it through experimenting and helpful advice from this board. Seems to be not nearly so sensitive to trim-in angle than previous boats, and needs only the gentlest touch of tabs, just before she climbs over the top.

As originally instructed, I'd been putting the damn things all the way down before nailing the throttles, and the effect was like pulling away with the handbrake on...

The A7's seem fine, and I wouldn't want to lose any top speed. Just wondering about C7's (the stainless equivalent)

 

[Scubadoo]

You seem to be thinking on the same lines as me. I considered S/S props last year and decided against it. You will get better performance from S/S props (that's what the prop expert told me).

I decided to leave it for the moment, you can't antifoul S/S and I wanted to try the new Veridan product which won't work on S/S. As the Veridan seems to work I will stick with Alum. The other worries are, S/S prop is unlikely to get damaged if you hit something, instead you have more risk damaging the gear box, and I also heard that your anodes need replacing more often.

My last boat had S/S and I was very pleased with it, and I may change me mind to S/S at some point.

I heard that all new boats Sealine, Fairline etc etc are now fitting S/S as standard (Volvo) as the Alum wear to quickly.

RM.

 

[Guest]

I've run stainless in the past on outboards and sterndrives. Yes, they take more damage, but require more expensive repair when it is time to clean them up. I've heard all that crap about damaging the gearbox, but frankly, if you hit an ally prop hard enough to rip the rubber bushing out, I don't fancy the gearbox's chances anyway.

What I did learn was that there is a wide variation in quality and performance of S/S props. Early Solas on an old Johnson: went like stink. No-name prop on a Mariner: no difference. Attwood Ballistic on a Mercruiser: slight loss of top end, but impossible to cause ventilation/cavitation - gripped the water like nothing I've driven.

Sadly, no-one I've ever met will let you try a prop out before you part with your cash...

 

[Guest]

Trying different props

On my Spirit 3000 I was considering switching over to S/S before I came across Pihrana props. These are composite plastic and the blades just slot into the hub. If you break a blade you just slot a new one in. The company marketing them over here, Hotshot Marine, allow you to exchange the blades until you find the optimum pitch/diameter. I went through five different combinations but ended with a better top speed and she got up onto the plane much more easily. I hope they're still available but I'm not sure what the current situation is. The used to be sold through Pumpkin and Southwater, the Mariah distributors. Pumpkin have ceased trading and Southwater are out of that line of business.

Doug