Shark Cat - Removing the wave-breaker? To do or not to do?

[Greg Woulfe]

Hi guys,

I am currently working on an 8m Shark Cat, removing the inboard engines, reconstructing the transom and putting outboards on the back... no problem, I've done a few already over the years and they turned out pretty good. The guy I'm doing this one for however, wants me to cut out the wave-breaker? Why, I asked? He reakons that it makes it handle like a shopping trolley at high speed, which doesn't make sense to me because it's not even touching the water at high speed? Anyway.... as the customer is always right, I'm still very reluctant to do it, as Bruce Harris designed it in for a reason and I can't come up with any technical answer as to why we shouldn't cut it out? Can anyone help me out here? Any advice greatly appreciated.

 

[Mr Efficiency]

G'day Greg, I may be wrong, but I expect that really wasn't put in there as a "wave breaker", but as a fairing behind which a duoprop drive for a diesel engine was installed, there were a few out there with the single diesel, but pretty slow with something like a 20 knot cruise speed. I assume it is moulded in, and not detachable. Tinkering with that would be more than a small job, and could have structural implications. It is a little puzzling as to how it could be causing the boat to track unpredictably, the typical shark cat handling was more like an aircraft carrier than a shopping trolley, with slow helm response. I'm a bit surprised that this one has had twin sterndrives in each side, could it originally have had a single centre engine ? I'm not sure if they used a "fattened" hull version of that boat different to the outboard powered boats. I've just not seen many with twin sterndrives, there were a few around with twin jets, but required a sponsorship by a fuel company.

 

[Mr Efficiency]

I'd be surprised if one came out of the factory, with twin sterndrives, in a hull with the centre fairing. It may be a single that has been re-engined, by someone who got sick of a sedate speed. You'd think the best plan would be to fit the outboards, and see if the handling issue arises, there is every chance the boat will rise somewhat more underway, than previously, with less weight. If still an issue, then the central pod can be removed then, if desired. I don't think I'd be touching it.

 

[Greg Woulfe]

I'd be surprised if one came out of the factory, with twin sterndrives, in a hull with the centre fairing. It may be a single that has been re-engined, by someone who got sick of a sedate speed. You'd think the best plan would be to fit the outboards, and see if the handling issue arises, there is every chance the boat will rise somewhat more underway, than previously, with less weight. If still an issue, then the central pod can be removed then, if desired. I don't think I'd be touching it.

I think you might be spot on. I will try to convince him to leave it there, lets reconstruct the transom, fit the outboards and then see how she handles. Thanks for your comments - much appreciated.

 

[upchurchmr]

Is it possible that in a cat this narrow, waves will pound the center section?
The wave breaker might be just that, given that at moderate speeds and in waves that area will have the bow waves from each side intefering/ raising the water level, then a wave will be added on top of that.

Could be pretty destructive at a given speed wave height combo.

Big touring power cats had a serious issue with the same problem, even though the width was very much bigger.

A question, I'm not a power guy.

 

[messabout]

When the boat had a single engine the wave breaker might have been an essential item. As Upchurch mentioned, the wave train from inside the two hulls will converge at some point depending on speed. If the convergence is at or near the centered prop, turbulence will seriously affect the thrust and probably cause ventilation. The wave breaker could have been a fix for that potential problem. With twin propulsion the whole deal is off. The props are not operating directly in the turbulent zone so that thing in the middle is not helpful.

 

[Mr Efficiency]

The great majority of these boats were twin outboard powered, no wave breaker of any kind, there was an afterthought to try a single diesel, that never really took off, because of sluggish performance, naturally that required a fairing ahead of the drive leg, and that is what is seen in the OP picture. I can vaguely recall hearing of some disaffection with the handling of these boats, the detail I have forgotten, I thought being restricted to a 20 knot cruise, in a boat that is capable of holding 30 knots offshore in relative comfort, was enough to mark it as a bad idea. A few commercial operators who didn't need the speed, used the diesel cat, if there was a need to have pot-hauling equipment aboard, and that required a boat quite stiff in the lateral plane, and the cat hull supplied that best in a trailer boat, where a monohull would not. Naturally, the diesel was chosen to keep the fuel bill down.

 

[Greg Woulfe]

When the boat had a single engine the wave breaker might have been an essential item. As Upchurch mentioned, the wave train from inside the two hulls will converge at some point depending on speed. If the convergence is at or near the centered prop, turbulence will seriously affect the thrust and probably cause ventilation. The wave breaker could have been a fix for that potential problem. With twin propulsion the whole deal is off. The props are not operating directly in the turbulent zone so that thing in the middle is not helpful.

That makes sense. I had not considered that at one stage it may have been a single engine design, but after your comments and comments from Mr Efficiency, you've both got me thinking that it probably was. I'll grind into the *** end a bit more and that should reveal any previous modifications. Thanks for your comments.