homemade rowing coaches launch

My sons and I built our "Ergs Don't Float" coaches launch from a 60s vintage 15' catamaran we found on Craigslist for $200. Designed it all in 3d and CNC cut the parts from ply and they fit great.

Had a couple training sessions now and overall it works pretty well. At low speeds it takes almost no throttle to keep up with the rowers when they're doing steady-state (slow) rowing so it seems really efficient and I can't even tell that I've burned any gas from the 3.2-gal tank after running it for 3 hours. Has a 4-stroke 20hp Mercury motor which works great. Quiet, easy to start, sips fuel, etc.

Right now I'm running it off a tiller for testing but later want to add cockpit steering and throttle.

When loaded with two people water sprays up pretty bad in front of the engine at full speed. Here's a shot of it loaded with one person and water still sprays up some but not nearly as bad as when really loaded but I think the drag is still limiting my speed even when light.

I think that water is hitting the round aft crossbar and possibly my 2"x6" transom board (deck doesn't have a bottom, just a 2'x6' frame with fiberglassed plywood decking) and causing problems there.

The commercial rowing launch catamarans have a protrusion that sticks down in front of midsection of the motor that helps with the water flow somehow (see pic). Forgot what this part is called but boat guys like their nautical terms so I'm sure one of you can refresh my memory.

What I want to do is make a T-shaped part that is a fairing for the crossmember, fills the gap in front of the transom, and also sticks down in front of the prop to provide better flow there. It will also add aft buoyancy to help my load distribution including when docking since if I step on the back it sinks. Though to be fair 90% of the time this launch will have one person in it and it's pretty well balanced as-is since I sit pretty far forward.

Suggestions on how to design the protrusion in front of the motor midsection? How wide? How far to stick down? Other tips to get the most out of our little trainer?

 

Cats with center mounted motors present some problems. Each of the hulls produce a bow wave and a secondary wave. At certain speeds the waves from the inside parts of the separate hulls converge. Too often they converge at or close to the leg of the motor. That can cause a lot of turbulence and spray and sometimes cavitation.

Consider mounting the motor on one or the other of the hull transoms. offset the weight by whatever means you can contrive...like the fuel tank or whatever. The idea is to get the lower unit and leg of the motor out of the convergent wave train.

A 20 horse motor is way more power than you need for such a boat when lightly loaded. The cat hulls that you have may or may not be from a derelict sailboat. If from a sailing boat the hulls will not be friendly to higher speeds.

If you increase the throttle to go faster, do the hulls squat? That is, do they tend to trim bow up stern down? Sailboat hulls usually have rockered bottoms which can cause the squat condition when at certain speeds.

 

I base my claim on experience from my own catamaran.
The underside of the pod must be flat, not rounded as it is right now.
The depth must be equal the antivent plate.
20 horsepower is a huge engine for such a small catamaran.
JS

 

It seems like the low aft crossarm is the biggest problem, plus 15 feet is very short for what you are asking of it. Maybe you could find a cheap Hobie 18 or Nacra, etc.
I've done fairings that attached to the leg of the motor and fairings that attach to the underside of the pod. The photos are of a rough-water boat, but the shot of the fairing shows that it's long (fore & aft). Without a fairing the water explodes when its hits the leg, starting at around 7 knots.
The boat in the photos goes 18 knots with the 20, so yours should go at least that fast.

 

A much better crew chase boat would be a simple flattie that is long and narrow. Such a boat could make only a small wake at low speed and will still be able to run well past 20 knots with your present engine. There are several advantages to a conventional, if skinny, skiff. For starters the coach, or other observers, can sit in the boat rather than on a trampoline. If need be, it will be a much more efficient rescue boat in the event of a capsize or other problem with the shell and its crew. Simple is Good.

 

What depth is the motor leg supposed to be submerged to? Is that what you are seeing? The unloaded photo shows your boat trimming very down at the stern, is the motor too heavy for it? Can you move the entire superstructure forward?

I echo the suggestions to try a smaller motor, if nothing else outboards are not really designed to operate for long periods at low revs - this will significantly increase your maintenance effort or reduce engine life.

 

The motor depth seems about right. When the person is sitting in the cockpit its pretty level. The pontoons are never fully submerged and don't seem to change attitude much during power changes when the person is sitting in the cockpit.

Though when I add the pod underneath it will add some aft buoyancy. Makes it sort of a trimaran- cool. I like F-boats and want to build one someday so this might be a teaser

Should I design the pod to be at the same depth as the plate above the prop, keep it pretty close to the prop mid section fore/aft, and fair it for low drag? So essentially it will move the plate effectively forward. About how wide of beam should it be to be effective but also not add gobs of drag?

 

A fairing should be only as wide as the leg of the motor and yes, it should only extend down to the upper cavitation plate. The motor leg is streamlined below that point. I still think that longer hulls would make it a much better and more capable boat.

 

Good info on the fairing, I'll add that plus fair in the cross member. Any tips on where to get a cheap cockpit throttle control and steering for this motor?

 

You reckon boat guys are bad?

When I was pulling sticky things back-and-forth on a very long and skinny pointy thing that floated on wet stuff, and had sort of metally-things that stuck out from the sidie-thing, I used to reckon they were a bunch of ******* because of the silly names. They didn't even have a key for the sticky things so what sort of lock was it anyway?

The person at the back who didn't do any work (they called them a cock or something, which seemed very rude to me) said I should spend more time "Errrrgggging" or something but whenever I tried to go "Urgggg!" it came out as "aaaarrgggh". And she lied all the time since I never caught one single crab no matter how often I shoved the sticky thing into the wet stuff.

I don't know if anyone who calls their sport after a way of drinking beer fast has any right to give another sport a hard time about silly words.

 

Russell, my club is considering something like an old Hobie 18 conversion to replace the old alloy runaboat rescue/start boat. We're on a small lake (400 acres but most of it is narrow arms that we don't race on) so we don't need really high speed. What sort of an outboard do you reckon would get us say 12 knots max. on a Hobie 18-style platform with a lightweight console fitted?

My completely uneducated gut feeling is that a Tohatsu 9.8 could do it easily; we have one that pushes our 28' 2000kg yacht to about 6 knots and another that pushes a little RIB to 15 or so. However, my gut hasn't had the experience yours has had.

We do very few rescues but I do think that the low freeboard of the 18 (or Nacra 5.2 etc, since Forbesie is working on a H18 revival here) hulls would be make it much easier when we actually do have to pull people aboard.

 

Yes, I think you would get 12 knots with a 9.8 hp. My old Grasshopper would do almost 16 knots with a Honda 15. You will still have to make a fairing for the leg of the motor and find a motor that allows remote shifting, throttle, and steering.

 

No one has mentioned the imaginative screen name that the OP is using.. His screen name applies for gasses, liquids and combinations thereof. Good on you (rho v^2 )/2.

 

Good catch. Also a rowing pun there around kinetic energy equation