How much horsepower??

[geoleo]

Been looking at some used Catalina s /Hunters most are 1980 s -90 s . 36footers 35.5ers . Im surprised that they were equipted with suchlow powered engines I see a 198 C36 with a 21 HP and others similar with 23HP. These are 6-8 ton boats and they must not go hull speed in a 3ft chop+ anyway I don't like to run any engine for extended hours at top speed... What gives here???

[Cheechako]

Not sure but it was a thing back then in some boats. My Hans Christian 38 had a 25-30 hp Yanmar! That boat was way underpowered. They will go hull speed in flat water, but challenged a bit in rougher water.

[john61ct]

MacGregor 26M or 26X will get you up at 20 knots with a 70+hp, super easy maintenance too.

[Nicholson58]

Camper & Nicholson 58, 40 tons, 115 Hp Westerbeke. It’s light in rough conditions.

[Wotname]

Originally Posted by geoleo

Been looking at some used Catalina s /Hunters most are 1980 s -90 s . 36footers 35.5ers . Im surprised that they were equipted with suchlow powered engines I see a 198 C36 with a 21 HP and others similar with 23HP. These are 6-8 ton boats and they must not go hull speed in a 3ft chop+ anyway I don't like to run any engine for extended hours at top speed... What gives here???

The engine was considered an auxiliary power source; the principle power source was the wind.

No point in fitting anything bigger than the minimum required to move you around when there was no wind.

Back in the 80s, we sailed sailing boats and we motored motor boats. If the wind was on the nose, we tacked upwind or waited for better weather or changed our destination to somewhere downwind.

Not saying it was better or worse, it was just the way things were done when sailing in the 80s.

[a64pilot]

A smaller motor burns less fuel, which means a smaller fuel tank, a smaller motor weighs less, and the smaller amount of fuel weighs less, it’s also, well smaller which means it takes up less space in the boat.

It’s actually tough to make a case for a bigger motor in a boat that’s meant to sail.

[Hasbeen]

My 40Ft Morgan Giles design had 8 an horsepower with a folding prop.


I did 53,000 nautical miles in her no problem, although not much more than 1000 would have been motoring. In the doldrums areas like the Solomon Islands, you could spend all day motoring along in the heat, or wait until nightfall brought a lovely off shore breeze, generated by the high mountains, & sail in comfort.


I always felt sorry for those who did not have enough confidence to sail their sailing boat.

[Dockhead]

Originally Posted by a64pilot

A smaller motor burns less fuel, which means a smaller fuel tank, a smaller motor weighs less, and the smaller amount of fuel weighs less, it’s also, well smaller which means it takes up less space in the boat.

It’s actually tough to make a case for a bigger motor in a boat that’s meant to sail.

Well, wait a minute, I think you are on record for stating the true fact that diesel engines generally burn nearly the same amount of fuel for a given amount of power produced.


The smaller motor burns less fuel only if you use less power; for the same power the difference will not be large.


A lot of power is one of those things you might not need often, but when you do need it, you REALLY need it.


The old rule was 4 to 5 hp per ton; I would want to be on the high side of that. My boat has 100hp for 20 metric tonnes light ship, so maybe 25 short tons loaded. I would never want less than that and wouldn't mind more.

[ttex]

My Baba weighs 10 tons and has a 32 hp Perkins 4108. Seems a bit much for a battery charger but hey what are ya gonna do.

[a64pilot]

Originally Posted by Dockhead

Well, wait a minute, I think you are on record for stating the true fact that diesel engines generally burn nearly the same amount of fuel for a given amount of power produced.


The smaller motor burns less fuel only if you use less power; for the same power the difference will not be large.


A lot of power is one of those things you might not need often, but when you do need it, you REALLY need it.


The old rule was 4 to 5 hp per ton; I would want to be on the high side of that. My boat has 100hp for 20 metric tonnes light ship, so maybe 25 short tons loaded. I would never want less than that and wouldn't mind more.

I am on record for saying that fuel burned is a factor of HP produced.
However if you have a 20 HP motor that kind of caps your max fuel burn at 20 HP doesn’t it?
If you have a 40 HP motor odds are your going to use more power than if you had a 20 and therefore burn more fuel.
Now I am not one of those that thinks you have to run the snot out of a motor for it to last, I’d say if you want your motor to run almost forever, run it at 50% power or less, so think of a 20 being a 10 and a 40 being a 20, but your still burning twice the fuel, but not going twice as fast.

However we have for some reason this belief that we need to make hull speed, I assume it started being in smooth flat water, now I guess there is a belief that you need of make hull speed pushing against wind and waves?
There is a name for that boat, Motorsailor, and there is nothing at all wrong with it if you want one, but it would seem that is where this idea of being able to make hull speed pushing against adverse conditions ends up?

Just for grins, my hull speed is right at 8 kts, and I motor at 6.5 kts in smooth flat water like the ICW, and occasionally I see sail boats blow past me, and I wonder why? However it seems that most motor about 6 to 7 knots, those that go faster are infrequent.

[Nicholson58]

All motors have a parasitic loss that is a function of size among other things. In short, a 30 hp motor producing 20 uses less fuel than a 100 hp motor producing 20. If fuel consumed per mile traveled is your primary concern then smaller is better. Last season we were motoring between Antigua and the reefs into current and 30 knots wind. Our SOG was less than a knot. By the calculations suggested above, my 40 ton boat should have 200 hp instead of the 115. I wished we had the hp. Running the 115 at high output only results in overheating even with an infinitely variable prop. If we ever wanted to return to Michigan by the St Lawrence it would be impossible. We motor 7.5 on flat water, no wind. Add head wind and chop and that drops to 4. As far as fuel is concerned, in 7 months cruising the Caribbean last year we used about 150 gallons including the 12.5 kw generator. Going to Windward we motor sail a lot. Fuel figures to be one of our least expenses. Feeding a bigger motor would not impact us enough to care.

[a64pilot]

That parasitic loss you speak of is friction, and at really large different sizes of motors it’s apparent, like your 20 to 100 maybe, but in motors closer to the same size, it’s not, say a 40 to a 60, assuming a 50% increase in displacement as of course sometimes they are the same block .

For example with an aircraft at lower altitudes and lower power settings you can make identical power at different RPM’s as you control prop speed. Well its a well known fact that if you slow a motor down and run it at higher manifold pressures to make the same power, you’ll use less fuel due to the prop being more efficient from less drag and the motor being more efficient from it having less drag.
I had a well instrumented little airplane, I knew individual cyl head temps and individual EGT’s and fuel flow down to the .1 GPH. It was all calibrated and was so accurate that if I let the airplane sit in the hanger all weekend all 14 thermocouples would read within 1 degree Fahrenheit.

Well to make a long story short, while I’m sure that lower RPM was more efficient, it’s logical and a well known fact, but on my IO-540W1A5D the efficiency difference was less than .1GPH, because my fuel flow meter read the same.

Just like it an accepted fact that if you run a motor lean of peak EGT, it’s more efficient than one being rich of peak EGT, I even had the special matched Gami fuel injectors to make lean of peak smooth etc.
Well when flown at identical air speeds, the fuel burn at the recommended Lycoming 50 degrees rich of peak was identical to being 25 lean of peak.

A lot of things are true, but when you actually measure the differences it may be so small as to require lab quality equipment to measure it.

But don’t forget that bigger motor both weighs more and takes up more room, from a clean sheet design that matters a lot.
Back I guess in the 70’s a an Engineering study was done by one of the major car manufacturers, and if at the drawing board stage of you left out the spare tire and jack etc, the car would weigh 500 lbs less and of course burn significantly less fuel.
What happened is if you took that stuff out, to have identical storage space etc the car could be slightly smaller, which didn’t require as big a motor or as big a transmission, or as big brakes, wheels etc.

Something similar may be the same on a boat, specifying a bigger motor on the drawing board will require a bigger fuel tank etc. and that has to make some difference in water tankage or performance etc.

I don’t know squat about race boats, but I bet that the ones that are required to have motors have tiny ones and little bitty fuel tanks?

Of course you have to balance that with being a cruising boat, are you willing to give up that small performance hit for a boat that can motor at hull speed without pushing the motor hard?
It would seem that looking at newer boats, the answer is yes, new buyers seem to want that ability, because it seems newer boats have larger motors than older ones.

[geoleo]

Originally Posted by Nicholson58

All motors have a parasitic loss that is a function of size among other things. In short, a 30 hp motor producing 20 uses less fuel than a 100 hp motor producing 20. If fuel consumed per mile traveled is your primary concern then smaller is better. Last season we were motoring between Antigua and the reefs into current and 30 knots wind. Our SOG was less than a knot. By the calculations suggested above, my 40 ton boat should have 200 hp instead of the 115. I wished we had the hp. Running the 115 at high output only results in overheating even with an infinitely variable prop. If we ever wanted to return to Michigan by the St Lawrence it would be impossible. We motor 7.5 on flat water, no wind. Add head wind and chop and that drops to 4. As far as fuel is concerned, in 7 months cruising the Caribbean last year we used about 150 gallons including the 12.5 kw generator. Going to Windward we motor sail a lot. Fuel figures to be one of our least expenses. Feeding a bigger motor would not impact us enough to care.

There might be a point in here but I cant find it.

[a64pilot]

Originally Posted by geoleo

There might be a point in here but I cant find it.

His point is to size an engine according to its use, don’t put a 100 HP engine in when and if a 20 is all that’s needed. It will among other things burn more fuel, it will also be heavier and larger, use more oil and most likely its parts will cost more too, and likely because it’s shoe horned in may be a real bear to maintain.

Actually he’s saying he wishes he had more than 115 I guess.

[HankOnthewater]

I noted that up-thread it was said: 4 to 5 HP per ton displacement. Does that mean that the old rule of 3 HP per ton is not valid anymore? My previous ferro boat had such ratio, and I was always happy with that. My current boat has also this ratio (as it is an older boat), and am OK with that as well.

Yes, there would be occasions that more HP would be better, but often there is limited room for a larger engine. However, when having a older type of boat, with an older type of engine, there IS room to install a newer common-rail engine that takes less space and provides more power.

I guess those rules ratios depend to a very large degree on how well the boat sails, and particularly how well it sails to windward, and also depends on 2 other factors: how well the skipper can sail/handle the boat, and how much the skipper is not in a hurry. If these last 3 elements apply, one can do with a smaller engine.....in my opinion.

Edit: but..... given the choice and all things being equal, I would fit the biggest engine possible, an engine that remains simple to maintain (and likely not common-rail)