What HP do I need for my boat?

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barryassheton

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Hi all,

You may have noticed I have another post on here asking people for information on their engine configurations so I can determine what would suit my boat but after doing some research there are apparently several formulas that can be used to determine what Horsepower I need however I was never good at maths and with all the different methods out there I am finding it somewhat confusing. So if anyone understands the math behind determining hp I would be grateful of some advice, so here is my boat specs

Senior 31
planing Hull
Approx 29ft at the water line, 32ft LOA
10ft Beam
approx. 4 ton with engines, full tanks etc,
the hull planes at 12½-14 Knts

I want to be able to get her on the plane and I dont intended to use her for distance trips just fishing trips and the odd trip the the mainland. Her intended use is around the Solent and Isle of Wight.

Thanks Baz
 
The Senior 31 is listed in my "25 Years of Motor Cruisers" book.
Two engine options,the first lists the boat being fitted with 2 x Perkins 50hp on outdrives giving 12 knots and mentions 16/17 knots been available with 75 or 80HP engines.
the hull is semi displacement with round bilges and a flattish shallow V section aft.
Around 1000 were produced many going for home completion.
There is also mentioned a Senior 32 Sheerline a different kettle of fish altogether.Based on an off shore powerboat it came with a pair of 180Hp Sabres on shafts of course which gave 25 knots.
 
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More info here.
25kts+ was definitely possible with the petrolhead option..
AQ171c's produced something like 168hp in top condition with a DOHC head. Not sure how much low down grunt they had, though.

http://www.jonesboatyard.co.uk/documents/Senior-31_1.pdf

senior_31.png


senior_31_2.png
 
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There you go then, twin 110hp. Don't overdo it as you don't want to over stress the old girl. My Senior 31 had twin Perkins 4107 (the worst engines in the history of crap diesel engines) and managed about nine knots when whey were running. The Seniors in the 1969. Round Britain had to withdraw as they started breaking up but that won't happen with sensible power outputs.
 
Thanks. I have a lot of documentation I found online. I even emailed Princess as their Project 31 and Princess 32 both use the same hull albeit with some modifications and they sent me a large pdf full of information. I am considering the Mercruiser 3.0 135hp, I am also considering the 4.3 V6 200hp although the Hp is a bit high it would never be used on full power. There is not much in it these days between diesel and petrol due to levelling of fuel prices and with petrol there is the option of LPG conversion I just need to find a way to make it viable as there is only one LPG station locally.
 
I would have thought the extra weight of the 4.3V6 isn't going to help with trim and 200hp per side is a bit OTT ... although they will sound nicer :)
Also, there will be more room to work around the smaller 4-cylinder engines.
 
FlowerPower said:
I would have thought the extra weight of the 4.3V6 isn't going to help with trim and 200hp per side is a bit OTT ... although they will sound nicer :)
Also, there will be more room to work around the smaller 4-cylinder engines.
Click to expand...

I had a look at the weight and the 4.3 v6's are about 50Kg heavier each than my previous engines I will have to work out space as there wasn't alot with the AQ171c's. I am having trouble finding data sheets for the mercruiser 3.0 and the 4.3 V6 Alpha one.
 
Similar ... MPI has fuel injection, more power (220hp vs 190hp), fractionally taller but same sort of thing.
190hp version has carbs.

mercv6.png
 
You need to be very very careful with LPG conversion.
In a car or automotive application, any leaks have the opportunity to vent away underneath the car, as the gas being heavier than air will always go down to the lowest point.

In a boat, the lowest point is the bilge, with no opportunity to naturally vent away it will collect and potentially form an explosive mixture. The slightest spark, e.g. from an electric switch, starter motor or even bilge blower could set it off and Good Night Vienna.
Petrol in boats is riskier than diesel for this reason, but is manageable with the right set up of bilge blowers and timers etc.
But LPG is another step up again in terms of risk, one I personally wouldn't be happy with at all.

Your choice of course, but be sure you have considered everything fully before you go ahead.
 
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