Engine HP size

Both the Volvo and Yanmar distributors down here said a rule of thumb was 4 HP per tonne but I think they are incented to go high.

I put in a 40HP for a 13tonne yacht - works a treat.

Craig
 
First consideration is whether boat is Motor-Sailer or engine is back-up ....

My 25ft motor-sailer has a 43HP Perkins 4-107 ....
My pals Centaur has a 12HP Volvo.
Another pal had a Colvic 26 with a 6HP Petter.
Another Colvic had a 12HP Petter Twin.

All these boats similar weight and size.
 
Opinion is certainly divided on this one! Naval architect Tom Colvin has said that for an ocean cruising boat you can go as low as half a horsepower per ton! He says this is because such a boat would only use its engine for battery charging and docking, and maybe getting through the doldrums. For coastal cruising he goes up to one horespower per ton because you have to punch the odd tide. In contrast, modern boat makers install around 4 to 5HP per ton. I wouldn't go cruising with less than 4HP per ton because I like predictable passage times and less reliance on the vagaries of weather and tide.
 
Thanks for all the help here, its going into a large steel Ketch that eventually is going ocean cruising, however its going to need a lot of shake downs round the uk coast first.
I think I will probably err on the side of caution and pick 4HP per ton, I like to think that I could make headway against the tide sometimes.
Thanks again for your help and advice.
 
I have some older magazines here, from the '60 & 70's. At this time, the rule of thumb was 3HP per ton. If you had 4HP per ton this was almost considered a motorsailer. Our present boat is approx 12 tons weight and has a Buhk DV 36 HP, which is equal to 3 times. I was a bit concerned about this when first purchasing the boat, as I thought that there wouldn't be enough hp to punch through wind & sea. We can push through, but very slowly and certainly extra horsepower would be welcome in this situation. On the other hand, it is a sailing boat, and if the wind is anything above force 3, then we should be sailing anyway.

However, what I didn't realise at the time, is the big advantage of having a smaller sizzed engine, is the huge extra range you get. This engine only burns lesss than 2 litres per hour @ 2200 rpm giving us a boat speed of 5knts in light conditions, but with a 350 litre tank we have a engine cruising range of over 700 nm. This was certainly appreciated when sailing the boat back from the Med to Ireland non stop last summer. We had to motor about 50% of the time into light headwinds. A larger engine would have meant carrying extra diesel or having to change course to pull in for diesel.
My tuppence worth.
Regards,
Declan.
 
This is a true story

My boat was built in 1937, displacement 10 tons.

According to Frank Knights, she was built with a 7 hp Kelvin E2. The first owner thought she was underpowered so he asked the builder, Claude Whisstock, to get hold of Kelvins and get a quote for the 15hp, E4, version. Kelvins wrote back saying "We have looked at the drawings and 7hp is quite sufficient for this boat. If the owner wishes to go faster we advise him to buy a motorcycle" (!)

Anyway, for the past 39 years she has had a 15hp Volvo MD2 engine, driving, incidentally, through the original Kelvin sterngear, now 69 years old!

The 1.5 hp per ton ratio allows us to just about pull the skin off a rice pudding. Motoring to windward is only practical in sheltered water, up to F5 or so. Top speed, in flat water and a calm, with a clean bottom, is around 3.4 knots.

On the other hand, it does give tremendous range on quite a small tank.

For coastal cruising a small engine does impose severe restrictions, which our parents and grandparents accepted but we tend not to. One must be very careful not to be tempted into a dangerous situation (which can be as mundane as crossing a bar) by other boats with high powered engines.
 
Very true! I have a Yanmar 2GM20 on a boat pushing five tons fully laden for cruising.
There is enough power unless motoring into fresh conditions when of course sailing
is more effective. Windage is a big factor on multis.
However picking up a MOB in gale conditions under engine only would be difficult.
It's also worth bearing in mind that
the manufacturers horsepower rating is often exaggerated. A 2GM20 prodeuces 16HP
at 3400 revs and even less at acceptable noise levels. However the range is great, and
weight saving both in fuel and metal!
It's also worth considering whether the boat is correctly propped and whether a self
pitching prop would make best use of your available power. Although I have just ordered
a Kiwi prop so I can have a three blader without killing the sailing perfomance.
 
When I started cruising in the early 70s the rule was 1 hp/ton for an auxiliary, 3 hp/ton for a motor sailer. Today it's more like 3 for an auxiliary and 6 for a motor sailer. At 10 I'm a bit OTT but on a multihull you can go on piling on horsepower and adding speed (within reason) whereas with most monos you're into hull speed limitations above 5-6. Anything less than 3 will leave you struggling to motor upwind in a blow.
 
sorry but the fuel consumption issue just doesn't sit right the way you have outlined it.

generally a diesel engine will consume fuel in proportion to the HP it is being asked to deliver and not it's size/rating per se. certainly in the sort of ranges you are discussing here.

so, for example, 12 ton boat fitted with either a 36 or 48 hp engine and making 6 knots through the water - fuel consumption should be identical for both engines (assuming they are correctly geared/proped and that 6 knots is well withing the displacement speed range of the boat and finally that the weight differences of the engines aren't in anyway a factor (which they won't be here)..............

yes the 48hp will at full throttle be capable of consuming 25% more fuel / hour but even here it would be the balance of the power to the hull / gearing etc that dictated whether this was translated into similar mpg or simply wasted as energy creating a wake (amongst other things!)
 
It also depends on hull form, gearing and prop size. I used to own a 50 ft MFV that was built as a sailing vessel - originally no engine - about 1914. When I bought her out of the fishing she was fitted with a 70 HP gardiner driving a massive 3 bladed prop and had worked for years as a seine netter with this engine. Her fine sailing boat lines made her the fastest boat of her size in the local fleet.
Don't know what her displacement was, but have seen much smaller motor sailers (Fishers come to mind) with far more horsepower.
 
4 horse per ton is what I've got (long keel displacement hull) and I'd say it was only just adequate. At 20' along the waterline, I need to be able to maintain (pretty much) hull speed under engine if I want to get anywhere in a decent time!
 
My NC 44 motorsailer has a 120hp 6.5l naturally aspirated diesel equating to about 7 hp/ton (17 ton) uses just under 2 gal per hr at 1750 revs doing 9.5 knots, engine alone, we've motorsailed at 12 knots with 800 revs under engine alone. At full chat (2300rpm) she'll do 12+ Kn in calm water. In flat water she's going at 4-5knots at tickover(400revs) the only way to keep control and get her to go slower when berting is to knock her in and out of gear. Kinetic energy is a wonderful thing.

I love the old fashioned nature of the engine, even if she is a mite overpowered, its relatively uncomplicated and not as high reving as some modern turbo diesels, with no engine managent and turbo's to go wrong. Its understressed for the work its asked to do, no bad thing IMHO.
 
Fuel use .... agree with Duncan ........ here\'s a table to illustrate

This table was compiled some time ago after I asked on the forums for data ....... I think those that haven't seen it may find it interesting and it supports the theory that fuel use is not dependent on engine size - but on work it does ......

fuel_consumption.jpg
 
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