Estimating Saildrive drag force?

Is it possible to estimate the drag force caused by a Saildrive with a folding propeller?
What I mean is in terms of performance loss.

In the PHRF fleet where I'm racing, a Sailboat with an inboard engine with a folding propeller would receive a benefit of 24 seconds per hour.
So what I'm asking is that if the drag caused by the Saildrive will be compensate by this amount in seconds.

Also, what is the best place to put the Saildrive in terms of distance behind the trailing edge of the keel?
This is a trapezoidal planform keel with a NACA 63-009 section, 960 mm root chord, 470 mm tip chord, 1150 mm span, and 24º leading swept back angle.

I think that the point where the flow is more turbulent will be the correct place but I don't know how to estimate the place.

In terms of weight placement, moving out the outboard with the transom bracket and replacing with a Saildrive engine that just weights 29 kg placed near the center of gravity is also an improvement.

I'm not just asking for technical details, any empirical answer will be much appreciated.
Kind Regards, and sorry for my English.

Román

 

I have heard that the most advantageous handicap for racing is to leave your trailer strapped to the boat.

Sorry, I just couldn't help it

 

The drive manufacturer may be able to give you an estimate of the drag on the drive unit over a range of speeds. Try asking them.
As far as location- you don't want it so close to the keel that it will screw with the flow off the keel's trailing edge. Most of the saildrives I've seen when the boats are up on blocks for winter are somewhere around midway between keel and rudder. That's hardly a scientific opinion though.
On a 63-009 section, unless the surface is covered with barnacles, the flow won't get turbulent enough to make much of a difference as far as the prop is concerned. I think the interior layout of your boat- where there's room for the engine- will be a pretty important factor in picking a spot. And that 29 kg is probably for the drive and gearbox without engine, right?

 

Thank you for your answer.

In a matter of fact, no. This is an ancient Volvo Penta MB2A/50S. 7,5 hp
It just weights 29 kg engine + Saildrive, it runs on Gas. It's a 4 stroke 2 cyl
Honda BF75 + a Volvo 50/s Saildrive.
Gori propellers still makes folding props for this engine...

My biggest concern is that, this is a 1/8 ton Sailboat, a scaled down version of the 1/4 Ton by Ron Holland "Manzanita", it's 7,05 mt LOA...
The displacement is 1065 kg.

Someone told me that it is too small for an inboard engine and the performance will suffer and doesn't compensate for the rating reduction.

Another guy advice me to put the Saildrive as close as possible to the trailing edge of the keel where the flow is more turbulent.

Any other help, will be much appreciated.

 

I had a larger yacht and used a 6HP long shaft outboard for a while. I eventually fitted an 8HP Bukh sail-drive and was very happy with the result. I am sure it weighed more than 29kg but I was still able to manhandle it into position.

Until I fitted the sail-drive I had to rely on sail in most conditions. After fitting the drive I could then motor or sail with good effect in heavy weather.

If you place the motor near the keel it will improve your sail carrying capacity and will not alter trim much. The extra righting moment will more than offset the slight increase in drag from the appendage and extra weight in all but the lightest of airs. It could actually sail better over a typical course.

One disadvantage is that you will be handling a flammable liquid below deck. I much prefer the safety and smell of diesel. (It is something I associate with boats) Also even at 29kg it is not that easy to take the motor out to work on like you can with an outboard.

Rick

 

I would instal this unit in the most practical position engine wise. The prop wash will aid steering. I have a quarter tonner with an outboard on the stern. Its a pain in any sort of seaway, prop easily leaves the water. When I leave the boat I place the fuel tank in the cabin. Some fuel fumes must leak from the tank. Obviously there is no spark to ignite this as is possible with a petrol motor inboard. Just have to be a bit cautious thats all.