Sadler 26 hull speed

[mick]

Does anyone know the above, and does the term imply that the boat will go no faster no matter what you do?

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[snowleopard]

hull speed is the speed at which the boat sits in a trough between the bow and stern waves and no amount of power can push it out of the hole. the formula for hull speed is 1.4 x sq root of lwl in feet = speed in knots. let's say your boat is 23 ft on the waterline, hull speed would be 6.7 knots.

the number 1.4 is a bit variable, a slim hull would be higher, a tubby one lower.

hull speed does not apply to long narrow lightweight hulls with length/beam ratios above about 8:1 which is why multihulls can exceed the theoretical limits.

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[Jacket]

The 1.4 also depends on displacement as much as beam. For displacement/length ratios of below about 140, the 1.4 becomes closer to 1.6. And then once you're below displacement/length of 100ish, planing becomes a possibility, after which there's no real theoretical upper limit to speed.

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[paulrossall]

Don't you also have to take the power of the engine into consideration. I was checking the size and pitch of my propellor as I do not seem to reach my theoretical hull speed. Propellor was ok. Boat weighs over 10,000 lbs and I was informed a rough guide is you need 2hp for evey 500lbs. I have a Yanmar 2GM20 which only gives me 16hp continuous (18hp peak). I really need >20hp.

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[snowleopard]

yes, i reckon a 2gm is a bit light for a 5 ton displacement, you'd be pushed to get up to hull speed with that power (but you could well do it under sail).

in my day (being an ancient mariner) it was reckoned 1hp/ton was ok for an auxiliary, 2 was luxury and 3 or more made it a motor sailer. today your 3hp/ton would be considered no more than adequate and my motor sailer has 10!

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[billmacfarlane]

Depending on the shape of the hull , you can be pushed along by a breaking wave which effectively planes the hull. Some boats do it more easily than others - it depends on the boat's displacement , hull shape and width of stern. The Sadler 26 is capable of it but it won't surf so easily as some of the more modern designs.

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[Jacket]

Engine power doesn't affect hull speed, its just a question of whether there's enough power to push the boat up to hull speed.

But remember, acheiving the last little bit of hull speed takes a lot of power. My boat with a 9hp engine gets up to 85% of hull speed. I've known a similar boat with an 18 hp engine that will do 95% of hull speed. Its just a question of whether you think the extra cost of the engine and fuel is worth the extra 0.6 of a knot.

To my way of thinking, your engine's pleanty big enough for the boat. Sure, it won't get it right up to hull speed, but it should get it close enough. Generally its only more moder boats, with their shallow hulls and broad sterns to prevent squatting that will get right up to hull speed under motor.

Out of interest, what's your waterline length, and how fast does the motor drive you, both at cruising revs and flat out?

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[paulrossall]

Waterline length is 26 foot, cruise at 5.5 knotts and just about 6 flat out. My hull at the stern is very narrow which does not help. I go from 9.5 ft max width down to 4.5 ft at stern. I am going to be more carefull this year with what I load into the boat. Just because I have loads of room I tend to take more than 1 kitchen sink!

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[qsiv]

Thats an intersting series of numbers - my boat has just under 3hp per ton (80hp on about 28 tons laden), and I've always described her as being a touch underpowered. We reach our hullspeed easily enough (modern hull shape), but in particular in very tight spaces, with a big rig and lots of windage and a feathering prop, I certainly have to allow for the lack of real punch at low speeds. It's probably as much prop as engine, but the ensemble 'feels' underpowered.

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[snowleopard]

don't times change! my first cruiser was a 6 ton gaffer. the engine was a single-cylinder petrol job with hand start and a little prop under one quarter. it got me home at 4 knots in a calm and helped picking up the mooring, apart from that we sailed.

in those days an 'auxiliary' was just that, a step up from rowing or warping alongside. it's just a question of expectations. today i want to be able to motor home on time and if it's a hundred miles upwind, so be it.

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[Jacket]

26 foot waterline on a Sadler 26? Are you sure?

The speeds you get sound reasonable- about half a knot faster than on my 24 footer.

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[qsiv]

indeed - but dont forget that there was also a great deal more space.

I was brought up in Newton Ferrers, on the River Yealm, and even in the '60s there was plenty of room to sail back to a mooring, and visitors could still anchor in the Pool.

My fathers previous boat (a 68ft Edwardian ocean racer) didnt even have an engine, and he would happily sail her back from regattas in Plymouth or Torbay and onto her mooring.

The big difference today is lack of room to manouevre - together with much more windage, and much less grip on the water, which all increase thye dependence on the engine to get us in and out of our berths.

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[paulrossall]

I did not say I had a Sadler 26, if you look at my profile you will see I have a Macwester 30. Compared to your boat I will be much heavier I think.

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[Jacket]

Whoops, sorry. It was the title to your post that confused me.

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