Inshore?

[Keith Hart]

The ratio I mentioned is simply the weight of the boat divided by the horsepower of the engine (in my case 50/15). So long as you do the same calculation for any boat then the ratio is directly comparable.

I suppose you really should add the weight of the engine, which in my case is 35 kilos. Therefore the total weight of boat and engine is 85 kilos. Now divide this by the hp, 15 and you get a ratio of 1 hp to every 5.6 kilos of weight

I was interested to see how this compared to the big ribs. I have catalogues which give the weight of the large and powerful engines, but I have no idea what the weight of the boats might be.

Keith Hart

[sasa]

The total weight of my boat and motor is some 600 kg. The motor has 90 hp which gives power/weight ratio of 1 hp : 6.67 kg. Top speed is 40.5 knots (according to GPS).
Of course, power/weight ratio alone is not enough to compare boat performances. Another extremely important factor is the hull efficiency. Power/weight ratio is only interesting when comparing very similar boats.

Sasa

[Keith Hart]

Oops! Bit of a mistake here, I looked in the wrong column. The weight of my boat is 35 kilos not 50. This makes the weight of boat + engine 73 kilos. Thefore the hp/weight ratio is in fact 4.86 kilos per 1 hp..

I quite agree sasa, but I was interested to see how the amount of engine power I have available compared to the weight of my boat lined up against the bigger ribs. From what you say it looks as though my little boat is not under powered! (15 hp is the maximum recommended size for my boat)

My interest centered not on the speed but on my boats ability to make headway against tide/wind/current.

The first engine I looked at was a 4hp Mariner. I'm glad that I got the larger engine. It certainly gets onto the plane quickly.

So am I correct in my assumption that, given I will only be 'inshore', my boat/engine combination is strong enough to have a good margin of power if things turned a bit nasty?

Keith Hart

PS My thoughts on this came on hearing the story of how my father-in-law was out on the Firth in his boat (10 feet, not an inflatable) with a 2.5hp Seagul engine. The wind turned and he was having great difficulty in getting back to point of departure. He received a tow from another boat.

[Jari Hjerppe]

With SIB's it's difficult to estimate the performance by thinking the weight ratio.
I had a Bombard Tropik 380 with 9.9 ob and now I have the Zod with 15 hp ob. Both boats are 3,8 m and the max speed is about the same (Bombard little bit faster!).
I think that the reason is the wet surface. Zodiac is wider and the v-angle of the hull (if there is any!) is less than in Bombard.
So it's difficult to compare soft boats with rigid one's.

Jari

[Keith Hart]

Hi there

I am not looking for a comparison really. What I am interested in is the ability of MY set up (see above) to cope with wind/tide/currents. Yes, it is SMALL but (here is where the comparison comes in), it seems to have a lot of power in ratio to the weight compared to the bigger RIBS. Okay it's small size will not help in heavy seas, but in smoother conditions, it should have enough power to cope against the wind/tide/currents.

Am I correct in this assumption?

Cheers

Keith Hart