engine size?

[CaptainK]

Dear Kai,

What kind of oil do you use in your nav lights?

And what size of propane bottle do you have on board Kittiwake?


Regards,


Kevin
CaptainK

[Alan Wheeler]

To answer your questions CSY. I'll start with No:3 first. No you can't go past hull speed for a displacement hull. Hull speed is the point where the wave length equalls the length of the hull. The length of the hull is measured as the "Loaded water line length". Now No:2 comes in to the play. You most likely aren't at Hull speed either. You can come close to it, but as you notch up towards hull speed, the required power to get you there, becomes exponential. So to answr No:1, yes you are approaching hull speed. As you get closer to that max speed, you require more and more power to gain small increments in speed. The Bow is trying to ride up over the Bow pressure wave and the midships is sinking into the trough causing the stern to squat down. The engine is being pressed very hard and will over heat and be burning huge amounts of fuel.
Hull speed is determined by the formulae 1.5 times the square root of the loaded waterline length, expressed in knots.
As for the prop, this is a difficult and technical area and the best situation is to get an expert to advise you. However, for a little info. First of all, you will notice there are huge numbers of designs of props. That's because there are many reasons to have a different prop design. What best suits the situation is a little more complex than this, but basicaly, a prop with a large blade surface area is designed for slower reving drive shafts, large HP and huge torque. They have enormouse coupling power to the water, but they also have huge losses of power due to water friction. A Prop with a small surface area is designed for higher reving shafts and require less HP and torque due to the lesser drag in the water. But they also have less grip in the water. Now it's all very well saying "I want a bigger engine" but it's kinda like that big V8 in a mini thing. It's no good having the big motor if you can't get the power to the ground. Boats are the same. And to spec a motor requires a little backwards working of the scenario. Firstly, you can only fit a certain size prop underneath. This is determined by the distance of the hull from the blade tips and the front and back diamensions of skeg and rudder. Once you have gone as big as you can, you have to have an engine and gear box ratio to suit it. Now, clarences may mean that you could fit a 200HP motor, but you wouldn't need that sort of power in a 30ft boat for instance. You can still only go hull speed. But if you need say a 35HP motor for the 30ft'er because of the weight formulae suggesting it's a good match and you can only fit a 20HP, no point in spending the money on HP you can't use. So you need to tie the HP/weight formulae against the "what can I fit" formulae as well.
Back to your prop question. Here's some ruff guide lines. Firstly, all engines have two RPM speed ratings. Maximum intermitent RPM and continuose RPM. The engine needs to be able to reach maximum RPM (or close to) but you don't want to run it up there. But you do want to be able to happily run the engine at it's rated continuose RPM without the engine labouring and overheating.
Hope this helps.

[BC Mike]

What Wheels said.
What would we need to do to get the boat to go faster given that we seem to hit a wall. The pointy end is trying to go uphill over the bow wave so the blunt end goes down.
Make the boat a lot lighter and make the blunt end a light wider. Make the run aft nearly flat. Add a huge amount of power. That is what they do with 16 foot fizz boats. To some extent that is what they do with trawler types. Trawlers may go 2 knots faster, than a sail boat, using a larger engine and consume a lot more fuel. Some may go 4 knots faster. I am thinking of 12 knots for a trawler and 8 knots for a sail boat, both boats at 40 feet.
We can not flatten the run aft without adding more surface area or reducing the weight, The run aft on my boat is fairly steep while it is fairly flat on a Laser 28. The Laser weighs about 4000 pounds and my boat weighs 7400 pounds.
We just can not have everything. If you want a solid cruiser type with good speed up to hull speed, then the moderate displacement ( and heavier ) boats work fine. If you want to plane, then the displacement length ratio, and the sail area to displacement numbers have to change, a lot.
Michael

[Starbuck]

I read Alan's caveat that limited his post to displacement hulls (which is, after all, the context from which CSY Man is asking his question). So in a very practical sense, it's very complete. He even uses the more modern 1.5 mulitplier that reflects advances in naval archetecture, rather than the old 1.34 that is found in the more classic literature.

Of course, some boats defeat the limits of their bow waves regularly: powerboats, planing dinghies, inflatable tenders with adequate horsepower, etc. Displacement hulls sometimes exceed their theoretical hull speeds when surfing down large swells in in extreme conditions (which may be a good illustration of the validity of the math: it takes an anomaly to urge a displacement hull beyond its hull speed).

Alan, I've always understood the hull speed, under the more normal circumstances we mostly sail under, to be an achievable reality: once there, the physics make further speed extremely pricey. But you seem to be describing it in a way much more like space travel using Einsteinian physics: that as you approach the speed of light, the mass of the vessel will increase exponentially toward Infinity so that one can never apply enough power to bring it up to C (the speed of light).

Am I reading you right? Is hull speed regularly achieved as displacement hulls push up against their bow waves using their normal available power sources; or is actual ship speed always a fraction of a never-attainable theoretical "hull speed"? My money is on the former, but I'd love to read your thoughts.

[Alan Wheeler]

Hull shape does of course have a big affect on what speed a boat is going to approach. But remember one important aspect, ALL yachts(errr, sailboats, we call them yachts down here) so ALL sailboats have a keel. A keel will cause friction and result in loss of speed. A keelboat of course, has a hull shape that would make it very diificult to support any lift. A flatter bottom boat with a fin keel can plane. Not under power, but I have planed a drop keel 25ft boat under spiniker. There is also a boat being built down here in NZ that raises it's keel and then gets up on the plane under engine power and hightails it at a real performance planing speed.

Kevin,
I use regular grocery store lamp oil for the nav lights, and the propane tank is small. 1 gallon. We just use the boat for weekend, 3 to 4 day trips, and that is more than enough.
As for a boat not being able to excede hull speed under power, Certain fin keel boats will plane under sail. I have been at 14 knots under sail on a 37 footer. Will these same hulls plane under power? I understand the amount of power needed to bring a boat up onto plane is exponentially higher than that to reach hull speed, but how much higher?

[Alan Wheeler]

No you won't get the hull planing, unless the motor is huge and the hull and motor package purposely built for such.
Well, this is where I am stepping out of my league. I was hoping Jeff may have stepped in to give us some knowledge. But I will stumble about and hopefully get close. Power to move the boat via sail is very different to power by motor. A whole compleately different set of dynamics take place. Firstly, via the way the power is applied. The sails pull the boat along as against push. But as they pull, they also lift. Then of course, you get lift from the Keel and rudder. The rudder alone can generate as much as 10% of the lift. Depending on wind direction to travel of boat, the boat maybe in a situation of Heel. This changes the water line length and places the keel and rudder more horizontal. Both giving lift and the effect of widening the hull presenting more planing surface. A down wind run under spiniker is just plain power developed by an enormuose amount of sail. Something has to give and it tends to be the boat.... untill it goes wrong.

Been there when it went wrong. Death rolls suck, and they break stuff. I understand the dynamics of sail power vs prop, and I understand how much lift is generated by sails, however, take , for instance a McGreggor 26. This is a planing hull under power, but will not plane under sail. (we have tried). The power on this boat is not huge, bigger then the average auxiliry, but not much, so what's the difference?

[CSY Man]

Aye Gentlemen, thanks for the responses and the comments on hull-speed and such for my porky CSY 33.

Yup, I am familiar with the old formulas and the thoughts on hull speed and displacement hulls, etc.

The reason I was asking if I was exceeding hull speed (Without 800 HP) is this: http://www.image-ination.com/sailcalc.html

The above web-page lets ya calculate 2 hull speeds, one under power and one under sail...For my short ship with a 25 feet waterline as per factory specs (More like 26+ fully loaded) the numbers are 6.7 and 7,6...A pretty big difference.

Sooo, If I run my big engine and my big prop at high power, perhaps I have exceeded the traditional hull speed @ 6.7 and going past 7.0 knots..?

That would explain a bunch-o-things.

[BC Mike]

Sails generate more horsepower when being pushed than when acting as a foil, and sails can generate more horsepower than the engine, unless you have a very large engine.
The numbers provide the answers, so look at the ratios as I suggested earlier.
My current boat does 6 to windward under sail, 6.5 under motor power, 7.5 on a reach and 8.5 downwind. All the above can vary a bit but you get the idea. This is a moderate displacement boat.
My lightweight 21 foot boat with keel down and rudder attached did 5.5 to windward under sail, 6.5 under power, 7 or 8 on a reach and we burried the knot meter downwind. That is past 12 and to the pin. With enough ballast we could plane on a broard reach with out the spinnaker.
The boat felt like it would plane with a bigger motor and my guess is the 15 hp might be enough and 25 definately enough.
When you look at planing hulls a few things are obvious, they are light for their length, they usually have wide transoms and they have a flat run aft, which they can do because they are light, and they have lots of sail.
Michael