Originally Posted by john connell
Thanks for the research
.You are spot on because I went around to my mates place who is offering me the Lister and got the manual off him.
It is a 46 hp.He still reckons the torque is the more important factor.
However the more I listen and ponder over what you and Borocay are advising it gets harder to discount going with a bigger motor.Not positive on the tonnage but it will probably be around at least 9-10
Am I correct about the engine weight? The site was UK so I assumed Long tons (but I could be wrong).
Your mate is correct, what's more important is the torque curve data (not the hp), the data hopefully is in your manual, it should read as rpm
(& hp) per “lbs-ft” or “nm” (most probably “lbs-ft” because its an older engine). If you've noticed, all the old Gardners have low hp (& low rpm) but higher torque (& larger displacement).
I wouldn't throw away the idea of the cheaper engine just yet. Like I said, you really need to get the torque data & your displacement
figures as close to accurate as possible.
From memory (& I could be wrong) the hp loss through the gearbox
is about 3%, also there's hp loss with the alternators (& any other attached devices i.e. pump's) – of which the loss is dependent on their size & loading.
The following paragraph's is my understanding of “displacement”. It is only from the basics of what I've been taught (commercial tickets) & what I've further learnt from reading etc. Having said that, there's some people here with knowledge of NA (naval architecture) design (MidLandOne perhaps?), perhaps they can correct me if I'm wrong
My understanding of “displacement”
is the total mass of sea water that is displaced by the vessel when fully laden - not the weight of the vessel (incl its ballast & full tankage weight). Due to different vessel design's, two vessel's that have the same internal volume - may in-fact displace different amounts of sea water. Therefore, if we have two vessels of 40' - one is a full displacement
with full keel & the other is light (or semi) displacement with a fin keel, then the fin keel will require less hp to go through the water (in clam conditions). However, the lighter fin keeler will have windage resistance issue's when the breeze picks up & will require more hp to keep speed.
So, asking someone else about their similar sized vessel may not be a good idea – because it may not give you an accurate displacement sum. The other point too, is that two similar length vessels of the same keel type may have different beams, different draft
, different internal weight (stuff inside), different external weight (stuff outside), different lines, different thickness steel
, different amount of bulkheads, frames & stringers etc etc. Without getting some original NA design data on your vessel, we don't know what it's accurate displacement will be.
Btw: sea water has a density of b/w 1.02 – 1.03 (depending on which ocean) of fresh water, therefore - each m3 of displacement will be b/w 1020 – 1030kgs (2248 – 2270 lbs).
Like I previously posted, an old rule
of thumb was 2hp per ton of vessel displacement (1 US short ton is 907.2 kg (2,000 lbs)). Like Boracay posted, once you start needing extra hp in weather
& to power other devices etc – then you'll need to seriously consider going to 4hp per ton. As an example, 15 metric tons = 16.53 US tons X 4hp = 66.12hp. Now, you could stick to the smaller hp engine & go slower – but you have to ask yourself what if you need the extra hp to get away from a lee shore or through a rip/current etc.
Finally, I'm not saying that the 46hp engine is no good, I'm saying let's get the torque data & the displacement data & see if it will match OK for your needs. If worse comes to worse, given you've already got a lot of tools, skills & location – maybe purchasing
a reputable design for a smaller vessel (30' ish?), sourcing your own steel & building it yourself (with the 46hp engine) might be a goer?
 I actually read once of a young bloke in the UK getting a Roberts design, sourcing his own steel
(because it was cheaper) & having it cnc cut for him (this doesn't mean that you couldn't cut it yourself from the design patterns). Edit: a design may only take into account the weight, dimensions & hp of certain engines (but it may be altered?)