Catamaran engine power

[Talbot]

I know other PSE have recorded 9 kts under sonic drive.

[exposure]

I have a very hard time believing 9 knots under power alone with no current. The Snowgoose Elite has to push too much water out of the way with the "planing wedges" . To go from 4.5 to 9 knots it would take a four fold increase in power, assuming you could overcome hull speed. The only time I see nine knots is under sail in a very stiff blow, or with the help of some current.

And the 38 HP I have is the largest engine I have seen on a snowgoose.

[Lightfin]

Sunspot, nice website and pictures.

[Talbot]

exposure once whispered in the wind:
I have a very hard time believing 9 knots under power alone with no current.

I was a bit surprised, but that was the quote in an advert. However I asked a chum who used to work for Vospers shipbuilding, completed his own Elite (beautiful it is too) and now is a consultant on a super yacht project - and this was his reply:

Best you can hope for is about 7.5-8 as they "squat down at the stern horribly under power after that due to the hull form. Different under sail as the rig is pulling rather than pushing if you see what I mean..... 38HP is 5 HP more than I have got and I get about 7 - 7.5. Don't know anyone who claims 9 with an Elite. The older SG may perhaps go faster as it is a lot lighter.

As far as prop is concerned it depends a lot on gearbox ratio and RPM.
I have a 4 cylinder Mitsubishi (Vetus) and have a 15x11 prop. Max rated HP 33 @3600RPM 27HP @ 3000RPM Gearbox ratio 1.8 to 1.

There is also a maximum rated torque that the Sillette leg will take which may limit his prop diameter.

Best thing to do is ring Sillette 081-337-8687 with all information and they will give you a recommendation. They are pretty good at it as they have a big database of Prout applications. You will have to buy a new prop from them anyway as they are Radice (Italian) splined props and they are the UK agent.

Obviously an email or fax would be more appropriate for you.

Hope that is of use.

Mark

[exposure]

Thanks for the good info Talbot. "Squat" is a good way to describe what happens. As I power up, the wake I see comes primarily from the "planing wedges" as they drop into the water. He is right that under sail she does much better. And, I have noticed that motor sailing really helps the speed, probably due to the moment from the sail lifting the stern. Your friend's comments help explain that.

That being said, I am probably being conservative with my speed numbers. Until two weeks ago I did have not have a working knot log. When I bought the boat I found it not working after I put her in the water. I installed all new instruments over a year ago, but only did a haul out to change the through hulls two weeks ago. Since then I have only made minor attemps to calibrate the knot log.

Up to this point I have been relying on the GPS. The currents around here are strong and almost always present, making an accurate "through the water" reading difficult.

[Talbot]

if all else fails, ask what speed other boats are doing, and that would give you a better idea. I actually have two logs - one is an old fashioned spinner driving through a bowden cable from a fitting on the outside of the hull. This helps me to compare the two and I soon know if the standard thru-hull needs to be cleaned of weed etc.

[Cool Change]

I have just changed engine from a 9.9 Yamaha 4 stroke to a 25 Zuzuki 2 stroke oil injected, on my 36' Prout Snowgoose. The 9.9 just didn't have what it took to motor into the wind and current. The 25 will move me at a respectable clip and only burn about 1 Gph.
Cool Change III
current at anchor in Miami.

[Talbot]

Do you have a high thrust prop? My catalac was originally fitted with a Yam 30hp, and after a lot of messing around I eventuially discovered that Yam did a high thrust prop. This has much larger blade area (although the same diameter), and a curled lip on the exhaust diffuser. The results were particularly impressive when going astern - the boat would actually stop! It also made a difference when pushing against a high wind.

Do you have wings on yours - I fitted a small set to mine, and again it is only bad weather that shows the difference, but it does make a difference. I went for the small ones as a compromise between what I really wanted, and the effect of the upthrust on the engine mounting.

[Bill Houlihan]

'99 Athena (Fountaine Pajot) 38 with 2 Volvo 18hp's gets me about 7 kts - assuming flat water and no current - measured on my GPS - so assume it's pretty accurate. I burn about 1 gph.
Into the wind, 15kts, and seas 3'-4', about 5.7-6 is about it. Wish I had the 28hp version!

Regards,

Bill
San Diego

[vincik]

Good day everybody,

the 38 HP is not the highest on a Prout 37 elite: my Snowgoose was fitted with the Volvo MD 2040, rated 40 hp on a Sonic outdrive. Prop is a 14 x 14 three blade.
Max. speed somewhere between 7.5 - 8 (when hulls are newly cleaned and painted) up to 3000 revs.
I had a long time problem with the engine ( which even an official Volvo dealer could not solve!) as it was (smoothly!) running on two cylinders, still giving me about 6 Knts.
Problem solved by very competent mechanic, so now speed is up.

Merry christmas!

[JusDreaming]

our boat weighs about 12.5k not counting all the stuff we have burdened her with And we have twin 18hp perkins diesel

Based on the info on this thread, and some VERY helpful input from Never Monday, I have purchased a 3YM30 for my tri. I am hoping for 8kts under power, but expect 6. Unlike you cat guys, I only have room for one screw, but 29hp should drive the boat pretty well. The launch is getting close, so we shall see.