Engine Offset

tsunami · 09 April 2001, 13:15

I would be interested to hear of any first hand knowledge anybody has on the topic of Engine Offset. Normally this type of thing only applies to large horsepower. I fitted a 225hp to my own boat and mounted it dead centre as nobody could give me a definite answer to my question. The boat runs very well but at times the boat tends to torque down slightly to port. Any comments on this subject would be greatly appreciated.

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Joe

jace · 09 April 2001, 16:47

I’ve found offset to be necessary when fitting single outboards to most sizes of Rib’s to counter act the prop effect. I don’t think there is a definite guide to how much offset to put on an outboard, really depends on hull shape against horsepower.

It’s more of a guess with unknown hull shapes and a bit of experience of fitting lots of outboards helps. Usually best to speak to the manufacture for advice.

For the 225hp if you put an offset of about one and a half inches to Starboard (at a guess without looking at the hull) you will find it will run truer especially on throttling down and wave jumping.

Powerboat · 09 April 2001, 20:50

Jace's reply ( an inch to an inch and a half sounds about right to me .You can also cant out twin installations with the legs out from each other . I understand that one South African Cat RIB maker has them canted out by as much as 20 degrees !

One of the bits of advice I always give to first time buyers is to buy the boat , engine and trailer from the boat manufacturer , who is the person who knows best how to hang an engine on his hull and how to set up the trailer . The boys over here in Lencraft have turned engine hanging into an art form .I am sure that most other makers are just as good on their own hulls .

You can always rectify pull by adjusting the engine trim tab ( also probably the anode ) to point slightly in the direction that the boat is pulling to with hands off of the wheel . i.e. if your boat pulls to Starboard , twist the trim tab out a tad to starboard so that the water flow hitting it forces the engine back towards port . A little bit of trial and error is needed and any adjustments required are normally surprisingly small . This measure is also very useful on larger ( 30 to 40 hp ) hand steered tiller engines to remove the pull on the helm at high speeds .

Best wishes ,

Stuart

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www.powerboat.org

Alan · 09 April 2001, 21:28

The 150hp on our BWM DS 21 is offset by about 1.5 inches. No torq steer and the boat lands flat and level when coming off a wave which I think is the main thing for a RIB. Danger without an offset engine is that the torque effect will twist the hull in mid air so that rather than landing on the V of the hull you land on one face with a consequently harder ride! Or at least thats the theory as far as I understand it. I suspect like most of these things its rather a black art! (Depth of V, planing pads etc can have an effect.) If your current set up works then don't mess!

Al.

tsunami · 10 April 2001, 08:11

Thanks everyone for your views on Engine Offset. One of the problems I had is the fact that I designed and built the boat myself so there was no existing boats to compare offsets on. Therefore I decided that it would be easier to mount it in the centre to start with and go from there.
Although it only torques down slightly I think I'll take your advice and offset it possibly the 1.5" and see how it goes. I'll keep you posted to let you know how I got on.

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Joe