Displacement to HP ratios

[Judders]

My new purchase has a 9.8hp o/b engine and displaces about 1800kgs. It's a big and heavy block and I would like to replace it with something smaller and easier to remove. What thoughts are there on power to displacement ratios? I always thought that 4hp was a bit OTT for my Foxcub but I once has to whack her to full throttle to make headway through the early flood into Chi Harbour.

 

[Pete7]

Does it work? if so leave it alone for a year and go sailing. You will know then if you really do need an engine of this size. The 6 - 15 hp 2 strokes weigh 31 kgs, but a 2 stroke 4hp still comes in at 20 kgs.

Pete

 

[Evadne]

My boat has about 2hp per ton (6.5hp and ~3 tons) and is rather underpowered. I certainly can't punch the full ebb tide at springs in Chichester, and struggle to get hull speed (5kts) in anything of a seaway. Other boats of the same class have found that 4hp per ton is adequate, though some are as high as 6hp per ton. Prop size matters as well, of course, and a 3-bladed prop would seem to give more oomph in adverse conditions than a 2-blader.

 

[seumask]

I have 25 ft boat of similar weight to the feeling 720 with a Yamaha 8hp (28 kg) 2 stroke outboard. I often wish I had a few more HP. Especially when trying to motor into a strong tide or significant head wind so I echo the stick with it for now thought. If I replace it my current favorite would be the Tohatsu 9.8 2 stroke, with its good power to weight ratio.

 

[seumask]

One other thought if you want to go racing you can always take it off and use your lightweight mariner/merc/tohat,etc. 3.3 instead that you may have for your tender.

 

[bazobeleza]

Beleza is 8bhp/ton and she will punch through most seas (at least all seas I'd ever want to be out in). Sailings great but theres no substitute for horses. /forums/images/graemlins/smile.gif /forums/images/graemlins/smile.gif

 

[sailorman]

3 > 4 hp per ton displacement is plenty
our old Co 32 under 2.5 hp per ton /forums/images/graemlins/frown.gif

 

[roly_voya]

How long is a peace of string! - It all depends on what you want to do under motor. The min is about 2hp/ton which is fine if you just want to manover around and out of habour. It should give a top speed of about 1xsq rout LWL on flat water with no wind. You will struggle in any significant breeze especially with a chop. About 3-4hp/ton is a 'standard' axillary, should give you hull speed (1.4xsq route LWL) on flat water and be fine up to about F3. If you want off shore motoring and be able to make good progress into wind in rough conditions then the standard for motor sailers was generally reckoned at about 6hp/ton. There is little point in anything above that as you will have to slow down or shake the boat & crew to bits once it gets abouve about F6 (for boats up toabout 40ft anyway). The smaller and lighter the boat the less sense it make to fit larger engins as they are not going to want to motor at full speed to windward even in moderate conditions. If you have higher than average windage, ie high topsides, big sprayscreens, lots of rigging etc then maybe increase the figuers a little.

The other issue is the prop. There is no piont in having 6hp/ton on an outboard, you need a deep 3 bladed prop turning slowly to put that amount of power into the water otherwise you will just cavitate and make foam and noise but no progress. 4hp/ton is about the limit on traditionally shaped boats like folkboats that dont tend to lift there sterns but probably 3hp/ton is about it for a modern lightweight, particularly if she has a broad transom & fine entry, boats like this give up quite quickly when the sea picks up at all that bouyancy in the stern lifts the prop out of the water.

There is a common misconseption that big engins will help pushing a tide, its a myth. Max speed OVER THE WATER is about 1.4xsq route water line length however much power you have so if that works out at 5kn and the tide is doing 6kn you may as well give up and drop the hook till it turns. Big engines only hepl against wind and waves.