2nd Sea Trial Composite Catamaran Honda BF 40's

[stevemid]

We got the tachometer's set to display correct revs and took another run in the cat and reached the proper rev range at WOT (thanks Garry.) We made 20 knots at 5000 RPM (exactly what the designer predicted) but I'd say a comfortable cruise speed with these motors would be 16 knots. There's no fuel flow data to verify though as it is not set up on the boat.

This link (thanks moonlighter) tells where and how to set the motor height vis the cavitation plate. http://www.veradoclub.com/smf/index.php?topic=137.0
In comparison to Verado site it seemed to me that our motor legs were burried, although it's hard to tell due to the turbulence back there. The cavitation plate at rest was just below the waterline but when we powered up, the motors dug in and the transom sank a further 50-70mm and stayed there. It did not come back up to the surface as with a normal planing monohull on reaching planing speed.

I've attached a couple pictures for reference and I'd appreciate comments especially from cat owners (and others knowledgable on the subject.)

1. Does the distance back from the transom that the pods put the motor make a difference in all this? In our case the pods put our motors a good 250mm back from the back of the transom. I understand water rises the further back you go.

2. Hull shape. I've had contradictory advice on whether a catamaran hull is a planing, semi-planing, semi-displacement, or a displacement hull. My designer insists it is a planing hull. There are 70mm wide planing strakes running down each side of the sponson. However, the Yammie expert tells me all cathedral hulls are displacement. Does it matter or should I put the motor height as per the verado website?

3. Could the fact that the transom's dug in that 50-70mm and stayed there indicative that the motors don't have enough torque? or again doens't it matter, just raise the farking thing.

4. Turbulence. There's that much turbulence around the motor, that I can't tell quite how high I have to go. The picture is a still image I captured from a video, so I apologise for the quality. However, is this amount of turbulence normal? I know that on the Verado pictures and on my 16' tinnie with a 40HP Johnson, the exit from the transom was clean as a whistle, just like the Verado site. This just looks messy to me.

Thanks guys for any comments.
Steve

[Chimo]

Hi Steve

IMHO the motors need to come up and the boat would also perform differently (better) with more HP. More HP if you dont go too heavy will in all probability be equally if not more frugal at better speeds and the hull will probably sit better too

Here are the results of a search on here that you may find useful http://www.ausfish.com.au/vforum/sea...archid=1087230

Also one of these yields very interesting advice and results ie

9-01-2011 09:03 PM #54
Dean1

Join Date
Apr 2007

Re: Repowered 5.2 KC with twin 60 suzuki's.

Mate with the motor trimmed level with the bottom of the hull you need 65-70mm from the bottom of the hull to the cav plate. Your motors need to be raised one more hole do you have another hole left?? This is great news as you will get better performance again! Did you try the other props?


30-06-2011 08:59 PM #64
Chris20984

Re: Repowered 5.2 KC with twin 60 suzuki's.

Hey everybody, Good news, i finally got time to do some work to the boat.

Trip 1.

Lifted the motors up as high as they could go with the existing holes which puts the cav plate at 55 mm above the bottom of the hull. went for a run, could see 70 % of the cav plate. throws a big roster tail when trimmed up 3/4 or more. Had the 14x11 3/8 props on, two blokes, gear and full petrol. Super responsive on the throttle, up to cruise speed in seconds and hit the rev limiter pretty quick at 28 kts. getting 1.35 km/ltr

Trip 2.

Kept the motors at the same height, put the 15 x 11 1/4 props on, three blokes gear, full petrol. flat out the motors Didn't hit the rev limiter, about 300 revs under, only got to 28 kts. comfy cruise on 22 kts. much less responsive but still OK. fuel burn much the same.

Trip 3.

Got the drill out and drilled another set of holes in each pod higher than the originals, lifted the motors up to 70 mm above the bottom of the hull. Kept the 15 inch props on and went for a run with three blokes, full petrol and gear. could easily see 99 % of the cav plate on top of the water, cruise on 22 kts, hit rev limiter at 30 kts.

Thanks to everyone that helped me. I will post some pics soon, Cheers Chris.



Cheers
Chimo

[stevemid]

Hi Steve

IMHO the motors need to come up and the boat would also perform differently (better) with more HP. More HP if you dont go too heavyCheers
Chimo

Chimo you Beauty!
Reading through that thread, Chris20984 ran into and solved the EXACT problems I saw with Franks's boat and which I'm sure to have with mine (since the pods are designed to put the cav plate level with the bottom of the hulls.) This ,however, this gives me the confidence to alter my pods now while it is still in the shed!

On the HP issue I'm sure you're right there too and everyone on here says the same. The 60 suzuki's at 108KG or the 70 Yammies at 117kg are going to be the go!

Ok so here's the new dilemma. I built my pods 5" (125mm) higher than the plans planning on putting on motors with an extra long (25") leg However, the base line is still with the cav plate level with the bottom of the hull so I won't have enough adjustment holes to raise the motors 70mm. The way it's looking I could buy long (20") leg motors instead (in which case I'll be 30mm high) , or possibly I can just drill my first set of holes 20-30mm higher than they should be. That will give me 70-80mm headroom to raise.

Steve

[Chimo]

Or Steve you can mount plates set back off your pods to give you room for nuts and bolts off the motors and adjust to your hearts content and really get it correct.

We have a guy up here who builds such things if your interested?

Cheers
Chimo

[stevemid]

Chimo, yes I would be interested. The backs of my pods are 2X25mm ply laminated together. I had already been sketching out a 2nd 50mm plate that would bolt into the original holes and give me another 50mm of adjustment (100mm total). I'm very conscious of extra weight that far back.... What would that manufactured plate look like? Can you PM me a contact? or just respond here if that's legal.

Steve

[Chimo]

Check your email Steve

C
C

[gofishin]

...However, the base line is still with the cav plate level with the bottom of the hull....

. Spot on Steve. With pods, i.e. setting the motors further aft, this base line raises approx 1" for every foot length of pod from memory. However, the shape/dead rise of the hull, as well as the stance of the hull when planing, also has a lot to do with how quickly the water returns to its normal state/level, hence height of your motor. Props/line of thrust too low will cause the stern to dig in and bow to raise. You need to get the boat to 18 - 20kn and hang way out the back to eyeball what is going on with the A/V plate.

..... or possibly I can just drill my first set of holes 20-30mm higher than they should be....

. Yep, that's what I would do. 25" legs, transom top and holes set/drilled 20-25 higher than normal for a 25" donk. Props can also play a huge part in being able to set the motors at the right height.

If you can, play with the height of motors on this other boat, find their right height & transfer height theory to yours. Remember also that the exact leg height varies a little between manufactures.
Cheers
Brendon

[chris69]

Hi there Steve looking at the hull pic and the second motor pic too me it seems to be getting turbulence from the water leaveing the hull and hitting the pods.

As far as it being a displacment or semi displacement hull i dont think so, definatly planning hull,you dont make a displacement hull out of end grain balsa,

and yes raise the motors a hole and see what happens then,cheers chris.