switching to twins! - need prop advice

[johnrambo]

So, I currently have a 1998 zodiac hurricane 530. It was originally equipped with twin 40hp yam 2 strokes. I got it with a 2004 mercury 4 stroke 75 hp. It is a 20 in leg with a 17p 3 blade aluminum prop.

I know the boat was supposed to have a long leg outboard but this is the engine that was on it when it came from coast guard. The boat does have a planing pad at the bottom of the deep V, so with the engine all the way down it was not to bad height wise. It ran well but it would blow out if I trimmed out more then a few degrees.

Last summer i was getting 32 kts flat out with 3 adults, 2 kids, a dog, fishing gear and 18 gallons of gas on board. Went pretty much the same speed with just me and 6 gallons of gas on board. I think I could have got more speed out of it had I been able to trim out more. Its a big torquy outboard for a 75 (415lbs, actually the merc 115 block), neck snapping acceleration and hole shot. I was going to try it with a 19p prop until...

I procured a nice set of barely used fresh water 2009 Honda bf50's. 20 in leg, no props. Going to convert the boat back to twins. Combined they are lighter then the merc (205lbs each), are correct leg length for twins, and 25 more hp total. So I'm hoping to get equal or better performance. Would like to get 38-40 kts.

So how do I prop these?

When I use the Honda prop calculator it recommends 11x15p or 10.75x16p 3 blade props (inputs: 1200lbs hull, general use, offshore hull shape, 500lbs load with 18 gallons of gas, twin 50 hp engines).

With 2.08 gear ratio, this would give me a maximum theoretical speed of
33kts at 15p and 35kts at 16p. In reality should be a quite a bit slower due to prop slip. Seems low.

Should I just go to 19p like I was going to with the old engine?

[johnrambo]

19p prop would give me a max theoretical speed of 41.6 kts.

[95gstnj]

I'd definitely do stainless props. If you hope to be able to plane on one, mount motors a hole lower than you think and pitch props a little less than you think. I had a hurricane 533 barely, barely plane with 2 ppl with one 40 merc 4s. Did 19kt wot.

Twins are the way to go but I don't know if you'll see that much of an increase in speed even if propped for it. I'd start with 17p if I was doing it.

Let us know how it goes!

[Pikey Dave]

I'd concur with the 17p, I ran twin 115 Etecs on a 6.4 (difficult to draw comparisons I know) with 19p Mercury rev4 props. It would just about plane on one engine. I think 19p would be too tall on your setup. The 16p from the Honda calculator might not be a million miles off.

[johnrambo]

thanks guys. Yes maybe the Honda prop calculator is taking into account being able to plane on one engine.

My father ran one years ago with twin yam 40 two strokes and he said it was no trouble to plane on one engine with 3 people on board.

I've seen people running these boats on single yamaha 4 stroke 50hp (and etec 50hp) motors, they did not seem to have any trouble planning. No idea what prop pitch they were running though.

Been doing some digging on ribnet and it seems like most 5.5m ribs going 35kts+ are running 19p and up.

[jwalker]

We need more info...
What diameter of prop are you currently running?
Are the blade shapes similar to the Honda props?
What is the maximum revs for your current 75hp engine and what is it's gear ratio?
What is the maximum revs for the Honda engines?

In general, with the proviso that the current prop is the same diameter as the proposed Honda props then: If you're using 17pitch now and that is correctly pitched for your boat, you'll need more pitch for your twins or they will over rev.

But, given the Hondas are less power each my gut feeling is to start around 17" pitch.

[beamishken]

Isnt the usual advice with twin engines not to attempt to plane with only one engine? If a boat is set up for twins and your only running 1 engine it will be massively over propped and risk possible engine damage trying to plane on 1 engine. Personally id prop for best performance with 2 engines and accept if one goes down your coming home at displacement speed.

[Pikey Dave]

Originally Posted by beamishken

Isnt the usual advice with twin engines not to attempt to plane with only one engine? If a boat is set up for twins and your only running 1 engine it will be massively over propped and risk possible engine damage trying to plane on 1 engine. Personally id prop for best performance with 2 engines and accept if one goes down your coming home at displacement speed.

I can only speak from personal experience, my twins had 19" 4 blade props. As a pair, the holeshot was impressive at all speeds, bury the throttles at 4500 & it leapt up to 5750. They both topped out at 5750 which was the WOT sweet spot. On a single, it struggled up on to the plane & if you had more than 2 pax on board, you had to move a couple forward until it got over the hump, but once it was up, it was quite happy to stay there.

[pagick]

The other members who replied clearly have more experience with your setup and boat. My futura just got a 60 hp etec and it came with a 17p. But I have just started my prop size search and picked up a eBay ProPulse Adjustable prop 15p to 21p 4 blade. For me being able to change pitch depending on load and activity would be great if it performs well enough. If it performs like a plastic prop then at least I can better understand what pitch prop/ props to have and I’ll put it back on eBay. And it is only $99. US dollars
I’m running it next week.