How much horsepower?

[Dockhead]

Originally Posted by kenbo

On your next boat with a Hundested, make sure you also fit an exhaust gas pyrometer. That way you can tweak the prop pitch and the revs to the sweet spot for your particular engine.

Also you current boat seems to be around 4hp/tonne fully ladened, correct?

YES -- the pyrometer is essential with a manually variable prop, because you can overload it it really screw it up. This was explained to me some time ago, by someone on here, I believe.

I don't know exactly what my current boat weighs fully laden, but since "light ship" in this case, includes the generator and other equipment and half tanks, I am guessing it must be in the range of 22 metric tonnes or so. So I would guess a bit more than 4hp/t.

My boat is not overpowered by any means. I can make hull speed in reasonable conditions (9.3 knots), but I have wished for more power on occasion when trying to fight through bad conditions.

Part of my problem might be my prop -- Bruton Autoprop, which for all of its virtues apparently is not so great when you get near maximum power. It shifts down too much or something. So the same amount of power with a Hundested (and pyyometer!) might give me more thrust.

But I would like more power anyway -- with a turbocharged engine it's almost free power -- doesn't cost you weight -- just costs you possibly bigger marine gear and prop.

[john61ct]

Originally Posted by kenbo

Here's a question for the gearheads, do you size the prop to the max or continuous rating of the engine?

I would think having maximum extra power available for an emergency would be a good thing, never mind the redline.

Puttering along the other 99.9% of the time, maximum efficiency is (IMO) less critical.

But isn't that the natural order, mostly how it works anyway?

[kenbo]

Originally Posted by Dockhead

YES -- the pyrometer is essential with a manually variable prop, because you can overload it it really screw it up. This was explained to me some time ago, by someone on here, I believe.

I don't know exactly what my current boat weighs fully laden, but since "light ship" in this case, includes the generator and other equipment and half tanks, I am guessing it must be in the range of 22 metric tonnes or so. So I would guess a bit more than 4hp/t.

My boat is not overpowered by any means. I can make hull speed in reasonable conditions (9.3 knots), but I have wished for more power on occasion when trying to fight through bad conditions.

Part of my problem might be my prop -- Bruton Autoprop, which for all of its virtues apparently is not so great when you get near maximum power. It shifts down too much or something. So the same amount of power with a Hundested (and pyyometer!) might give me more thrust.

But I would like more power anyway -- with a turbocharged engine it's almost free power -- doesn't cost you weight -- just costs you possibly bigger marine gear and prop.

I have an Autoprop as well and I agree with high shaft rpm issues as well. But mine is not perfectly sized to my engine/gearbox so it's nice to know that someone else feels less enamored at high rpm.

I think Autoprop's claim to fame is lower rpm's at cruise and motorsailing and I appreciate those two aspects enough to minimize it's issues.

[Dockhead]

Originally Posted by kenbo

I have an Autoprop as well and I agree with high shaft rpm issues as well. But mine is not perfectly sized to my engine/gearbox so it's nice to know that someone else feels less enamored at high rpm.

I think Autoprop's claim to fame is lower rpm's at cruise and motorsailing and I appreciate those two aspects enough to minimize it's issues.

Agreed. Mine works great under most circumstances, and is also good in reverse and in harbor maneuvers, with very little prop walk and powerful thrust in reverse. But seems to fall down when I'm trying to get maximum power out of it.

[schaf]

Originally Posted by GTom

I see boats having very different HP-rating/displacement ratios - e.g. 5tons boat with 20 and another 5t boat with 40HP engine, is there a rule of thumb for the right engine size?

The goal is to cruise at 80% hull speed, allowing for some 30kts headwind (good reason for motoring...) and not-the-best bottom (=large wet surface from a full keel, some growth) without pushing the engine to its extremes (where the tank gets empty quite quickly). Obviously this seems more than the usual "get in/out of the marina" job...

Crusing the Pacific Northwest we deal with a lot of currents.
I always thought I would need at least 30 hp for my 30 ft 7.5 ton 3/4 keep. However I cruise at 5.5 it's burning less than 1 Ltd of fuel with my 10 hp Saab I have great range and don't have to fuel very often. Maintenance costs are minimal .

[Terra Nova]

Originally Posted by kenbo

...The theoretical hull speed formula is 1.34 x LWL1/2...

No. It is 1.34 x the square route of LWL, not 1/2 LWL.

[kenbo]

Thanks for that for those that don't know that any number to the 1/2 power is the square root. Notice I did not type LWL * 1/2. It's standard maths expression, but perhaps some readers didn't get it so thanks for clarifying.

[hellosailor]

There are formulas, and I think marine architect David Gerr has a book covering that along with propeller sizing, for properly powering a boat. I'd also note that a lot of brokers describe a very a very popular "3xxx" series engine as 30hp, but the maker rates it as only 24hp for sustained service with something like 1 hour at 27 hp before it starts to scream and throw tantrums.

A friend's father spent WW2 in engine rooms for the USN and remarked that the USN rated (required) all of their engines for continuous duty hp, with all engines being able to run at 3x the rated hp during combat operations. (i.e., when they didn't really give a damn about longevity, they needed survival.)

Remember that loads like an alternator also steal hp from the engine.

[kenbo]

Originally Posted by hellosailor

There are formulas, and I think marine architect David Gerr has a book covering that along with propeller sizing, for properly powering a boat. I'd also note that a lot of brokers describe a very a very popular "3xxx" series engine as 30hp, but the maker rates it as only 24hp for sustained service with something like 1 hour at 27 hp before it starts to scream and throw tantrums.

A friend's father spent WW2 in engine rooms for the USN and remarked that the USN rated (required) all of their engines for continuous duty hp, with all engines being able to run at 3x the rated hp during combat operations. (i.e., when they didn't really give a damn about longevity, they needed survival.)

Remember that loads like an alternator also steal hp from the engine.

This is probably thread drift but would you size your prop for Max rated HP or continuous HP rating? I've thought a bit about this but can see pros and cons to each choice.

[hellosailor]

Prop sizing is always a bit of "ballpark" and guesswork. I think it is really only done properly when someone says "Gimme about four or five of those and I'll see what really works." Something in theory a builder does with Hull#1.

I'd probably go for something that was optimal for the entire cruising+ range, if possible. If I was anticipating a lot of heavy weather sailing, I'd optimize for greatest load at greatest sustained speed, knowing that was the time I might need the engine most, and everything else was just "convenience" compared to that need.

[kenbo]

The downside to that approach is that you would lose all the available HP above the continuous rating. Fixed props are simple and trouble free but they are very crude at getting the most out your variable speed engine. Similar to a single speed bicycle, do you gear for the flats or the hills???