Which Outboard ?

[Fanie]

I'm looking out for twin outboards for the motorsailer boxy fisher, 10m.

Since you guys have more experience with outboards on the sea, pls advise me which are the best ones, and even more so, which ones do I stay away from.

The size I'm looking at is in the 28hp to 50hp range. Also if you feel it is too big or too small, say your say.

The temptation is there to fit two 150's eh

Thanks

[safewalrus]

waste of time fitting anything much bigger than what'll drive ee at hull speed, just easts gas (or petrol)! What do they use most of in your area? Well thems the best to fit, no matter what they are (easy to maintain and plenty of spares if everybody uses 'em, if your the only one in the area well............stands to reason really) Unless of course you are going further afield (do some research about your new area, see above!)

[Fanie]

Ok, I understand that the displacement hull has a ratio that determines it's max speed. Do we have a table to look that up in ?

Personally I don't like running my outboard(s) at max revs, but prefer half throttle where everything works comfortably and fuel consumtion is much better as well.

'In my area' the motors are mostly all >80hp and most are 115's to 200's, and they are used on fresh water dams, but they are mostly Mercury or Yamaha.

Assume some bad weather/wind. Even though the outboards may not be able to push beyond the hull's speed limit, wouldn't some extra power be a help in bad weather or won't it make a difference ?

[marshmat]

Without seeing the hull or knowing its weight and configuration, it's hard to estimate how much power it will need to go how fast.
The curious grammar of the Walrus aside, he is dead right. With most of the major manufacturers being very close in terms of technology, efficiency and performance, one of the most critical factors is maintenance. Can you get parts when it breaks, and can you get them quickly and cheaply? On this front, you're best off with brands that are popular locally and that your dealers like to service.

[Fanie]

For what it's worth, this is what the hull looks like.

It is 10m long and 1m200 wide.

As I indicated, the idea of having a slightly bigger motor would not be to get the hulls to plain or go faster than the hull speed allows, but rather to have some extra power in bad weather / winds.

I seem to remember seeing a boat that the guy couldn't turn into the wind as the wind was pushing harder than the motor (granted, it was a single motor). If you're in open water you could make up speed downwind and then turn, but if you're close to rocks you're in trouble - I don't want this to happen.

Oh, I forgot the weight. I haven't worked the weight out exactly since I haven't decided on the material I would use, but thumb suck it's going to be around 500kg max, not more.

[Fanie]

A rough calculation got me around a basic weight of the hull alone without anything in it, 288kg's

[marshmat]

That hull looks like it will be efficient enough that I don't think you'll have to worry about reaching hull speed. I'd agree with you that you should size the engine for beating into the wind if necessary in the worst sea state you'd be operating her in.
The displacement number is barely legible in that picture. Does it read 0.55 tonnes?
(edit) - if the hull is 288 kg empty, and given the criteria you've outlined, I might figure on a single 30 hp, or perhaps twin 15s, thus allowing 100 kg or so for power. Seems like a starting point at least.

[Fanie]

Displacement seems like it's 1.123 tonnes - The weight is just the hull's, but there are also a cabin, deck and aft deck and the beams that adds up on top of that. I think it would be even more than that considering all the rubish one takes along...

I was thinking more in the range of 2 x 30's at least ?

[Fanie]

I do seem to remember from many years ago, the 10's and 15's were extremely light on fuel.

[safewalrus]

Fanie if you want to add extra power 'just in case' (which is after all sensible) you don't want to add to much! I mean if 15 horse will drive the boat at 'hull speed' 30 horse (double) is a waste of fuel and money both if you run at full tilt or rev down to go normal speed! So as Marshmat sez 30 or two 15s will do the job make that say a 40 or two 20's (actually two 20's is better if one goes 'tits up' you still got the other one to get you home! Alternatively hows about a 30 to get you out and back and a 10 for trolling around! even more economical on fuel! But primarily check out the other guys and copy them - but why the hell they need all that power (smuggling maybe? or big boats or just plain show offs with more money than sense)!

[marshmat]

I was thinking 30-ish total... the resistance hump at hull speed on that thing will be small, and my guess is it'll start at around 7 knots (but it'll be gentle; you won't see the huge spike in resistance the fat-butt cabin cruisers do); you could probably cruise comfortably at 5 knots with a 9.9 kicker but that would leave no buffer for wind/waves.

[Fanie]

Thank you for your replies, very much appreciated !

I have recalculated the total weight of the 2 hulls, cabin, deck and aft deck and two beams - it is the bare cat without motors etc on it, comes to 2432kg give or take a few kg's, but for safety's sake looking at 2500kg empty. Adding the extra length, from 8m originally to 10 meters makes a huge difference.

Taking the extra weight in consideration, will the 15's, 20's or 30's still be sufficient ?

[marshmat]

Hang on. We just went from 288 kg to 500 kg 1123 kg to 2432 kg empty. We went from looking at a half-tonne ship to a three-tonne ship. Previous guesswork must now be discarded.
Fanie, it looks from the drawing like you are using Freeship/Delftship. Displacement hull, high length/beam ratio, no freaky weirdness. Perfect candidate for a few Michlet runs, if you ask me, to get a ballpark estimate of what kind of resistance we're talking about.

[masalai]

Fanie, Where are you proposing to go in that design, and what range, & how far offshore?

Regards

[Fanie]

I feel really bad about the faulty information I gave initially, I beg your forgiveness. Due to various unknowns at the time I had to do some changes. The information at the time however was what I had as the actual weight.

I am in fact using Deftship.

Masalai, at the rate things go here in SA I may build my boat, pack my stuff and bugger off forever to anywhere else Seems like a bunch of people here have plans.

You're asking questions I have very little experience about. I have been on a boat on the sea only one time, so my experience is limited to plenty on fresh water. I do read all I get to, watch the youtube vids especially the bad weather ones and try to form an idea of what to expect out there.

From all that I concluded the bigger the better, hence the changes to the larger cat. I don't even know where the thing is going to sit here ! I just know this is something I have to do.

The main idea is a motorized sailer cat, so I can sail it or use the outboards and it is to go fishing with once or three times a year. I always had a thing for water... never could stay off or out of it. Away from it for some time I get pullout symptoms.

If possible, I don't want to set too much of a limit.