Too small?

[nicksharpe]

I've got a 3.8m SIB which usually gets pushed along by a 20hp 4-stroke Mercury.

It's very heavy, however, and we're going down to Windermere for a week soon. Speed limit being what it is I'm thinking of getting a smaller engine for the trip.

Question is: what's too small? We're two adults and two four year old kids. Lots of ~2.5hp engines out there reasonably cheap - would they be enough to push us along?

Thanks

Nick

[chipko]

Originally Posted by nicksharpe

Lots of ~2.5hp engines out there reasonably cheap - would they be enough to push us along?

Probably. We had/have a 2.3 Honda for an aux. motor. It will push a 3.5/3.8 sib along at 6mph regardless of payload and throttle. From half throttle to full it just gets noisier but still goes at 6mph max. Great little motor. [emoji106]

[Fenlander]

Agree with Chipko... anything around 2.5-4hp would be fine for displacement speeds.

For such use we keep a 25yr old Mariner 4hp. So easy to carry after the 20hp and a basic reliable motor. Pulled it out at the weekend after 18mths untouched and it started 2nd pull.

[Rover310]

I'm running a 1991 Evinrude 6 HP two-stroke on my Avon Rover 310 Aero, which works well with two adults and one 4 year old aboard at displacement speeds, but I don't think I'd want a motor any smaller than 4 HP. If for no other reason than to be able to get off the water quickly should a sudden storm come up.

In fact if I can find a nice clean one with low hours, I want to upgrade to a late 90's, early 2000 9.9 HP two-stroke... I'm finding those motors are hard to come by in my neck of the woods though.

[The Gurnard]

Hi Nick

Displacement speed in knots is roughly worked out at 1.34 x square route of the boat length in feet.

So displacement speed which is the fastest your boat can go without going on the plane is approx 4.5 knots... which for me ... on an inland loch is fine.

There is a fair bit of windage in a SIB with 2 adults..so to be on the absolute safe side..I would suggest heading into the wind on your outward journey..so if the wind gets up..it helps blow you back the way you came. A 2.5HP could have a job pushing your setup into a strongish wind with some waves.

This photo will perhaps give you an idea .. me in this boat with main OB (its switch off) boat is 4.3m long and must weight around 400- 450kg..Its a 3.3hp aux that is pushing it along very nicely around 4.5 knots. I have seen a few nice looking second hand 3.3hp for sale lately around 200 – 250 quid mark..but its just your luck if there is one when you are looking

If that speed and considering wind directions suits your needs..go for it. You cant get blown far in an inland loch...even if the wind does blow up.

[Cliff B]

A couple of summers ago I had a Waveco 2.3 (slated floor) with a 2008 Tohatsu 3.5 4 stroke. I used to take that out at Southend and it struggled if the tide was running anything more than about 3 knts with just me and my ten year old son in it.

As soon as I got my T40 I put the 3.5 up for sale. Although it was a fantastic little motor I didn't feel it would even be enough as an aux.

[lightning]

What about a Tohatsu 9.8 two stroke?

Only weighs 26kg (the same as the 4hp four strokes) and will probably still plane with your 3.8m if you wanted it to.
At slower speeds 4/5mph it's fairly quiet as it is only doing 1500-2000rpm.

[beamishken]

I'm with the gurnard on this one a 3.3 if you can find one or a yam /mariner 4 or 5 would be all you need plenty about and pretty cheep anything bigger will be a waste as your not realistically going to go any faster unless your on the plane & then your over the speed limit