Formula 40 singlehanded trimaran build log

Discussion in 'Multihulls' started by Corley, Aug 24, 2011.

  1. Corley
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    Corley epoxy coated

    That's fine, no problems I'd be interested to hear more about your F40 cat is it the early offshore type F40, they had to have some very basic accommodation if I recall correctly? What name did it race under when in the circuit? What do you use for an auxiliary? an outboard on a bracket? What height is the mast on your cat? Any photos you can share of your boat?

    Kurt's F40 trimaran had an 8hp longshaft outboard for auxiliary I cant see that being adequate in my area so am hoping to leave a bit of weight budget to allow a larger outboard.
     
  2. hump101
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    hump101 Senior Member

    Mine is the 3rd of the 1986 Irens/JTA cats that campaigned as "GAJ Neptune", now "Lunosa".

    At the moment we have a 10hp yamaha fourstroke outboard on a bracket, with a suitable prop. Plenty of thrust in calm conditions, but with the amount of windage it is not adequate in more than 30kts. The previous owner has modified the outboard so it steers with the rudders, which apparently improved things enormously (I've no experience of it without this mod - it must have been terrible!). The problem is not thrust, per se, but that the bow of the boat, once off the wind, blows further off, and unless the motor is steerable the rudders alone can't get the bow back onto the wind. The boat motored up to the quay when we bought it against a 110kmh wind (not local velocity, but the meteo figure, I would say we had max 40kts with a bit more in the gusts up the mast), and it was OK to make forward progress. I intend to play with this more once we are back in the water this season (next couple of weeks, i hope) as I think with a bit more board down it will be better behaved, but in any case I am changing the propulsion system as a part of other work we are doing., and I will increase thrust at the same time. We have significant current around our mooring.

    Mast is 18.2m, so about 19.5m above the WL.

    Volume in the hulls is actually pretty large, though apart from a rudimentary galley they are empty.

    I've got lots photo's, but if you want anything specific just let me know and I can pop down and take one.
     

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  3. Corley
    Joined: Oct 2009
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    Corley epoxy coated

    Thanks for the information I'm thinking somewhere around a 15hp outboard, that's what most of the larger club multihulls use it's a similar situation on the F40 trimaran lightweight but lots of windage I'm considering a retractable bow thruster as well would help with maneuvering in close quarters. My outboard is steerable from the offset beam bracket but I like the idea of linking to the rudder and would not be particularly difficult as the tiller and outboard are fairly close together.

    Sweet boat by the way looks fast just sitting on its mooring. I'll take you up on the offer of additional photos as I proceed through the build I'm sure there will be lots of details I'll want to check out.
     
  4. hump101
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    hump101 Senior Member

    No problem, just shout when you need something. 15hp would be about right, I would think. I'm fitting an electric drive system in a pod, which will give a total of 10kw on two thrusters, both retractable, but that won't be for another year.

    The problem isn't power, but that the thrust is not directed over the rudder, so steering is very poor unless the engine is also steerable. I will explore this more shortly and let you know. I think, with one daggerboard down to act as a pivot, control will be much better even with the 10hp. Your tri should be better still, as it should turn easier.
     
  5. Corley
    Joined: Oct 2009
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    Corley epoxy coated

    The daggerboard is complete. It's coated in interprotect awaiting final polishing. After fairing came in at 40kg pretty good weight wise we could have saved a bit more weight with carbon but the $ would have gone up exponentially. Saving the budget for carbon in the beams and rig where it will do the most good. Board is balsa core with sacrificial foam tip. Did I mention it's BIG still coming to grips with how much larger everything is on a 40 footer versus a 30 footer. It's still a manageable size though which is good.

    While reflecting in the boat shed Ben and I agreed that we will build the case full height mainly because Cat 1 requires no openings from the case into the interior of the hull. Just simpler to make the case in one piece although it complicates the design of the removable crashblock slot fairing that is made to be easily removable through the half height case.
     

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  6. hump101
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    hump101 Senior Member

    I promised an update on the behaviour of the boat under power when we launched, which we finally did at the weekend (another summer down the drain of over-work!).

    With the 10hp at half throttle we were making 5 kts in no current and into a F2 gusting F4 breeze. We had a worst of 2.5kts of current, which commensurately reduced speed by a similar amount, suggesting that air drag at these low windspeeds is trivial. Due to other issues (newness, narrow channel, lots of traffic) we didn't use more throttle.

    We've modified the engine steering so we now have full lock-to-lock on the outboard, and the handling was not problematic. The bow blows off in the gusts simply because the boat pivots around the rudders, but dropping one daggerboard by half a metre cures that by giving the engine some leverage about the pivot point.

    The one issue we found is that, in only small waves, the pitching is enough to aerate the prop (mounted 0.5m aft of the aft beam). I think this, more than lack of power, would be the most serious issue in stronger winds/bigger seas. This is exacerbated by the boat being very light at the moment (-400kg) which raises the prop and results in a form with less pitch damping than when immersed to the design waterline.
     
  7. Blackburn
    Joined: May 2013
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    Blackburn Senior Member

    ^^^

    Sounds like you could use a bigger engine with a long shaft?

    The main reason to have the engine on board is for getting out of a dangerous situation and save the boat when for on or another reason you may be unable to sail, in more wind, sea, and/or current, isn't it?
     
  8. hump101
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    hump101 Senior Member

    Absolutely, though the engine already has the longest shaft available and is mounted such that it is well below the aft beam when running (quite vulnerable to a soaking already).

    I notice from the new Yamaha specs that the 25hp model is the same weight as my older 10hp, so there wouldn't be a weight penalty to increase power, if the beam and mount could handle the extra thrust (beam yes, mount probably no!). However, I want to mount the engine nearer to the LCF so it is not affected so much by pitching, so some work to do.
     
  9. CutOnce

    CutOnce Previous Member

    Anyone honest would answer yes to this.

    --
    CutOnce
     
  10. Corley
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    Corley epoxy coated

    I've considered a small centrally located sail drive diesel on my project but being a racing boat I'm reluctant to add the extra weight or drag. Team Australia have gone for a retractable shaft drive it seems like the best option as it removes the drag issue but is costly. So in the end you seem to come back to the outboard which while not ideal does a passable job and on multihulls can be Cat 1 compliant.
     
  11. Skyak
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    Skyak Senior Member

    My 29 ft trimaran has an old 15hp 2 stroke in a well in front of the rudder. It seems to be the right size but it the water it gets is very disturbed and the thrust is smack into the rudder.

    Since your purpose is safety you're not looking for motor efficiency. Placement and reliability are what you need. Have you considered a hydraulic drive? The motor is safe and sound in the hull and you could put a big powerful prop (or two) as deep as you like.
     
  12. Corley
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    Corley epoxy coated

    I've not seen a hydraulic unit yet that is really lightweight and there is a weight penalty with having a hydraulic pump most owners who I've heard of who have them are not very happy with them. I'd be interested to hear of positive experiences though. Most endorsed options seem to be outboard or inboard diesel with sail or shaft drive.

    I've seen some electric saildrive legs that seem reasonably lightweight but once again reports from the field are not especially encouraging on the reliability front. It's true they were on a cruising boat where you would expect them to get more running time than on a racing multihull but the results were not impressive.
     
  13. hump101
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    hump101 Senior Member

    Only experience is larger hydraulic drive thrusters on offshore vessels. They are almost all replaced with electric drives these days. Hydraulics are not efficient, messy, and relatively maintenance-intensive compared to electric. Safer in hazardous environments, though that isn't as applicable to yachts unless you put the drive in the galley.

    With electric, I'm not sure about lightweight. The legs/motors can be, but by the time you have the battery bank and a generator, it is a heavy option next to an outboard. We are going to fit an electric system for other reasons, and we'll size the generator to match the minimum safe cruising speed, rather than max motor capacity, but even so it will be twice the weight of the existing outboard.
     
  14. Skyak
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    Skyak Senior Member

    My thought was one motor, two drive props. I would expect the result to cost a little less, be less efficient, have same or shorter maintenance intervals, but be a little lighter, deliver more thrust and control and be less likely to fail in adverse conditions, than a pair of outboards on legs. This is based on general engineering not up to date marine experience. Electric is improving but it needs time to prove reliability.

    Have you ruled out outboard wells inside the hulls ahead of the rudders? Some pain in and out but I think it might be best for your race results.
     

  15. Corley
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    Corley epoxy coated

    ^^ The problem with integrating wells is weight it's not a great deal of additional weight but an outboard on the rear beam on a bracket is about as light as you can reasonably go. There is very little gap between platform weight and displacement in this design so every bit of weight matters. Fortunately the Mini 650's have pioneered a lot of useful technology that make offshore racing possible with very low payload capacity.
     
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