Engine size

[Samurai_Dave]

I've got a boat that seems to be close to a Shetland 535 with a different cabin design. Its 17' long and 6' beam, fibreglass hull.

What size engine should I be looking for to give me a cruising speed of 15 knots? I've been told a 30hp would be too low powered to get it on the plane, even a 40hp might not be up to it. I'm considering a 50hp, would that be overkill?

 

[VicS]

Do not exceed the manufacturers recommended Hp... It could well invalidate your insurance!

The max recommended engine size for a Shetland 535 is 65 Hp http://www.shetlandowners.co.uk/shetland/viewtopic.php?f=14&t=282

 

[William_H]

Engine HP

As Vic says manufacturers say what the max HP is. On the other hand like a sail boat quite low horsepower will give a few knots. Actual power needed to get it to plane to 15knots depends on overall weight and fore and aft trim. Of course you can not comfortably do even 15knots in rough (choppy) water.
So eventually the decision depends on your wallet and your ego. (don't laugh). IMHO it is like owning a Frerrari in inner London but people do!!!
I was driving a 6 metre RIB with a 60HP Mariner 4stroke EFI during the week. I did try just for the heck of it opening the throttle to near full. I am not used to fast boats and I can tell you I did not enjoy the speed in slightly rough water. IMHO this boat was grossly over powered. But then I have a 6hp o/b which I reckon is just right for my 21ft boat. sail forever olewill

 

[Csail]

I would say 50 or 60.

 

[Coaster]

I was driving a 6 metre RIB with a 60HP Mariner 4stroke EFI during the week. I did try just for the heck of it opening the throttle to near full. I am not used to fast boats and I can tell you I did not enjoy the speed in slightly rough water. IMHO this boat was grossly over powered...

60hp seems very modest for a 20' RIB. Take for example the Humber Ocean Pro 6m RIB:

Recommended maximum power 175hp.

P.S. not recommended for a Shetland 535.

 

[rob2]

It's certainly true that you need a different mindset for powerboats than sail. A properly trimmed RIB can cut through quite a chop if it has a deep-V hull form. Leaping off waves can be fun but isn't going to do anything for your actual speed. I guess most users would suggest that 60HP on a 6m RIB is more like the minimum required!

Rob.

P.S. You wouldn't think it overpowered it it were coming to pull you out of the drink!

 

[Dabhand]

I used to have a Microplus at around 16 1/2 feet so fairly similar to yours. It came with a 25hp Johnson, much too small, probably got about 10 - 12 knots with a following wind and swell.
Changed it for a 50hp Mercury 2 stroke and could get 23 knots, measured with a GPS, in calm, no tide conditions and only me aboard. I generally cruised at about 17 knots comfortably and fairly ecomically.
If you have passengers then even a forty will have to work hard and hence heavy on fuel, I personally would aim for a less stressed 50hp, or even a 60 as the boat appears to be rated for this size.

 

[pvb]

I'm considering a 50hp, would that be overkill?

No, it's probably a minimum. In a previous life, I had a Shetland 570 with a 40hp outboard, and it wouldn't plane easily. I'd go for 50 or 60hp.

 

[Lakesailor]

I had a Shetland Sheltie with a 65HP Johnson. It would plane easily with 4 or 5 on board.

More recently I had a 15ft Taylor (sold as a Fletcher in the UK) with a 70hp which would plane very easily.

I'd put the biggest recommended motor on and just don't open the throttle unless you need to. The only proviso being that if you want to poodle about at 6 knots for long periods you're probably wasting fuel and it may not like running at little more than tickover unless well sorted.