RL24, RL28, and RL34 Trailable Yachts
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Transom modification to reduce drag on RL28 when sailing.
I am floating an idea to reduce the drag caused by water inside the outboard well on an RL28. Currently on Swan there is a bottom cover that is a real pain to get into place and lock down and then remove. In fact I don’t use it. I have thought about improving the current locking method of 4 barrel bolds but the whole exercise is still difficult as it disrupts helming. I also have real concerns that in an emergency that it would take way too long under pressure to get the cover unlocked and out and the motor back down into the water.
The main problem is that the water comes through under the boat as lamina flow and then hits the lower part of the transom in the well and lifts and swirls thus taking energy away from boat speed. Guessing it may lose 1/2 a knot as well as creating sufficient noise to spoil the serenity of sailing. If I cut away as much of the transom that hung down into the well as possible then the water would continue to flow through and maintain lamina flow no (or less water) swirling and less noise. I have yet to make any real measurements but I believe that cutting away to the base of the lower rudder support would do the trick (150-200mm possibly). It is possible that by doing this the strength of the transom and in particular the rudder support may be compromised. The solution may be to fabricate an aluminium strip (spar)or even angle bolted to the transom just above the new outboard well opening to which the rudder support could be welded or bolted. This spar would need to be of sufficient length to go across either the whole transom or a reasonable length to tie it together.
A possible added advantage is that the complex lifting and tilting arrangement required for the large Honda 15 (which also takes up heaps of room in the cockpit) could be modified to allow simple tilting to at least get the propeller free of the water as it would stick out just behind the transom. I can’t see this costing a fortune although there is some work involved in glassing around the cut and fabrication of the alloy strip.
As I said, no measurements taken yet but I am wondering if anyone else has thought about this method or any other comments.
Russell Rogers9-Aug-2008    Edit    Delete 
Re: Transom modification to reduce drag on RL28 when sailing.
Russell, I am not sure how similar the RL28 well is to my RL24 Mk 3. I also considered structural modifications but decided against it. I have fabricated a 2 piece wooden 'plug' that uses the hull cutout shape to keep it in position, a wooden wedge between the 2 peieces to keep it tight and the weight of the outboard (bottom of the outboard shaft rests on the rear piece of the plug) to keep in down. All 3 pieces are on wire safety lines that can be pulled in an emergency to remove it (also stops them disappearing during installation). I would not describe the seal around the plug/hull as perfect, but I wanted to stop the majority of the flow into the well. No impact on the rudder/tiller/outboard, but I only have a Honda 5 not a 15.
Alex9-Aug-2008    Edit    Delete 
Re: Transom modification to reduce drag on RL28 when sailing.
Why not put a smaller short shaft outboard in the well.

Skippers seem to be going for bigger outboards to replace smaller original outboards. I would suggest an 8hp outboard would be enough and the improved sailing might just make sailing in less wind viable.

If your worried about the noise with the smaller engine working harder, then get a tiller pilot.

Keeping the weight out of the stern so it is not submerged is just as important as closing the well off. Keeping the propeller out of the water is a pretty good start as well on many RL28s to improved performance and handling.
Greg10-Aug-2008    Edit    Delete 
Re: Transom modification to reduce drag on RL28 when sailing.
Thanks for the responses so far. They are valuable. Quick replies:

Alex, the outboard cannot sit in the well when tilted it is just too big. I guess the shaft is longer and possibly meatier than the original design allowed for. So any bottom plug, bomb bay doors etc cannot use the motor to hold them down they have to be clipped or bolted. When the motor us “up” it is on a hinge that pulls it up and forward into the cockpit.

Greg, You are absolutely correct if my main aim was sailing performance. But, one of the selling points for Swan, for me, was the higher HP outboard. The drawback was the actual size as the previous owner had to modify the well cowling to fit. My family background is power boating (people in Brisbane may know Rogers & Lough Marine Engineers) and whilst I am really getting into sailing I can’t see myself getting into racing in a big way but I see theRL28 as a compromise motor sailor. I envisage motor sailing occasionally in the southern end of Morton Bay where space is tight. Or for example, a couple of weeks ago when out with Keith M. we motored back at 6 to 7knts from St Helena to Manly as there was no wind at all and the motor kicking over nicely. And, when you need a motor for some type of emergency then it is nice to have a good size one. I would look at a shorter shaft outboard if I had to replace this one but is still quite new and I guess the longer shaft will stay underwater better in a sea.

Russell Rogers10-Aug-2008    Edit    Delete 
Re: Transom modification to reduce drag on RL28 when sailing.
Russel, your comments about bigger being better regarding the size of the outboard are common but not based on facts that I am aware of.

Displacement hulls like the RL28 and most yachts are limited to hull speed when travelling under most conditions. Certainly it would take perhaps 30-40 hp to plane an RL28 so any more than the power needed to achieve hull speed has little or no value.

In the 1980s I had a Southcoast Magnum with Yamaha's excellent 10hp 4stroke outboard. It was more than enough for the Magnum (or an RL28) and most owners of yachts in that class used either the 8hp or 10 hp outboards with good results.

Since then many of the original outboards have failed and new owners have shopped for replacements. I suspect salesmen have done a good job of promoting the attitude of bigger being better and wanting more generating power seem to have lead people to opt for 15hp outboards where 8hp was more than enough before. Of course the salesman wants to sell the bigger more expensive outboard so why should practicalities enter the buying decision.

What I am whitnessing now is many owners with these bigger heavier outboards are now finding the compromises.

There are some excellent 8hp outboards with high thrust propellors and decent charging systems available that may just be a better compromise than a big outboard that can't really use all that power.

There are people out there that will want 15hp to motor at hull speed into 45 knots or some such story. The reality is that those are survival conditions and speeds of 1-5 knots may be practical without pounding the hull or risking other damage. Even with 15 hp you are unlikely to use the power. Something like a modern 8hp or 10hp outboard should be more than enough for safe motoring.

Whats better an 8hp flatout or a 15hp at half throttle? If there are no practical uses for the 15hp, there is little differnce other than noise.

If you are towing a big tender in those conditions then a 15hp might be useful but then you stand the risk of damaging the rudder with all that thrust and steering loads trying to control the boats.

My suggestion is to see if you can borrow a modern 8hp shortshaft outboard and try it out, you might be pleasantly surprised.

Not sure what type of rudder you are using but if it is a drop rudder and you are motoring in very heavy conditions or even just at hull speed, consider raising it to half height to reduce tha blade area and reduce the forces on the rudder box and blade. It takes a bit more movement to steer but the forces on the gear are lower and less likely to cause a breakage.
greg10-Aug-2008    Edit    Delete 
Re: Transom modification to reduce drag on RL28 when sailing.
Thanks Greg I will take what you say on board. I guess I do come from the “size does matter fraternity” but also understand hull speed limitations. I might even allow myself to be convinced that a nice 4 stroke 10hp might do the trick if I was shopping for a new motor and it would possibly solve some space and tilt issues.
Russell Rogers11-Aug-2008    Edit    Delete 
Re: Transom modification to reduce drag on RL28 when sailing.
Russell, check out Peter Johannsen's RL28 motor tilting solution in the Tips and Modifications section. He uses a 10 HP short shaft Honda and an efficient bombay doors system.
Michael Oakley14-Aug-2008    Edit    Delete 

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