Will a "hydrofin" reduce transom strain?

[tjandrews]

My mother bought a 14 ft. aluminum boat in the early 60's. The plate is long gone, but I remember it was made by Traveller, and the model was "Corvair." It's a lightweight boat for its size, with the thinnest-gauge hull I've ever seen, and four aluminum seats. It has a vee hull, but as I am no expert I couldn't tell you if it is a semi-vee or deep-vee. My mother originally thought to use it as a cartopper, but it turned out to be just a bit too hard on the suspension of her '59 Chevy, so she bought a trailer. Two men, or three boys, can lift it onto the top of a car. My brothers and I did it when I was 13 and they were 10 and 11, but then we're farm boys and perhaps knew our strength better than others might.

The original plate had the maximum hp as 17, but when my father mounted a 15 on it once it bowed the transom back more than he liked. Of course, that was after the tree had fallen on it and flattened it, and we had it pounded back out, so the transom MAY have been weakened. After that we added a 2 x 4 that strengthened the transom a great deal, and a 9.5HP was fine on it for 20 years.

But this summer the transom board finally rotted to the point where it had to be replaced. I've replaced the original 1' plywood board with a piece of white oak, and replaced our 2 x 4 as well. To pull on it now, it feels stronger than it ever has.

At the same time, I have been given a '56 15HP Johnson outboard, which I have managed to bring back from the dead. I'd like to put the 15HP on this boat, and I think it'll take it now, but I think it would be a good idea to reduce the torque on the transom as much as possible without reducing performance. I have a Stingray Junior "hydrofin" mounted on my 9.5, mostly to give me better boat control at reduced speeds under windy, rough conditions. The boat tended to pick up at the bow and catch the wind at anything under 3/4 throttle with just me in it, and the fin kept it down for me. I'm wondering if the same effect would reduce the torque strain on the transom with the 15. It makes sense to me that it would, but then I'm no maritime engineer, either.

 

[Silvertip]

Re: Will a "hydrofin" reduce transom strain?

Lets not confuse the terms "torque" and "stress". Torque is twisting force on the transom while "stress" is a general term for "all forces". Since the fin or hydrofoil is designed to "lift", it does place additional stress on the transom. How much or how little trim you have the motor set for will determne if there is and how much additional "torque" or twisting of the transom there is. But observation under full or near full throttle will tell you whether or not the transom can handle the power of the 15 HP motor.

 

[Adjuster]

Re: Will a "hydrofin" reduce transom strain?

The fin will increase the forces on the transom.

 

[HappierWet]

Re: Will a "hydrofin" reduce transom strain?

The fin will increase the forces on the transom.

YES, but in the oppositr direction

 

[tjandrews]

Re: Will a "hydrofin" reduce transom strain?

YES, but in the oppositr direction

That's where my reasoning led me. The motor without the foil tends to lift the bow, meaning that it pulls back on the top of the transom and pushes on the bottom. ("torque.") Or perhaps more properly, the weight of the bow trying to fall back into the water does the pulling.

The foil lifts the stern, thus balancing the torque trying to lift the bow, with the resultant strain lower than without the foil - or so my brain tells me.

Of course, trim of the motor is critical, much more so than it is without the foil. At least, I found it to be that way during 10 years of experience with the foil mounted on the 9.5. Redistribute the weight by adding a person toward the front of the boat, and the trim has to be changed if things are to work right.

Again, the main reason for buying the foil in the first place was to keep the bow lower at sub-planing speeds with just one person in the boat, so the wind didn't blow it around. Without the foil the boat with the 9.5 was VERY difficult to control at anything between plane and almost dead slow if there was a strongish wind. In that regard, it functioned perfectly.

 

[Silvertip]

Re: Will a "hydrofin" reduce transom strain?

This discussion seems to indicate that if one put a big enough foil on the engine that there would be literally no stress on the transom. There is no free lunch here. Yes -- the force of the outboard pushing the boat tends to raise the bow and therefore twist the transom away from the boat. However, adding the foil forces the bow down only if the motor is trimmed down as well. Foils are generally added to avoid porpoising and to aid hole shot. The secondary effect (when purposely trimmed down) is to drive the bow down.. When the bow is plowing (or when the boat has more wetted surface) the same twisting force is present as you had before. One force holds you back (bow down) while another stresses the transom with the bow up. When you take the measurement -- if you really want to get into that amount of investigation, depends on how you have the motor trim set at the time.

 

[5150abf]

Re: Will a "hydrofin" reduce transom strain?

Yes it would increase stress, not decrease, you are adding something to the boat that influences the way it runs in the water, to do that there has be additional forces that weren't there before so with a foil there is more load on the transom.

So you have the original force of the motor pulling the transom back, that won't change, it takes the same force to push the boat through the water with or without a foil, so you won't affect the torsional forces on the transom at all but you will add a lifting force, so there are more forces acting on the transom with the foil.

 

[tjandrews]

Re: Will a "hydrofin" reduce transom strain?

This discussion seems to indicate that if one put a big enough foil on the engine that there would be literally no stress on the transom. There is no free lunch here. Yes -- the force of the outboard pushing the boat tends to raise the bow and therefore twist the transom away from the boat. However, adding the foil forces the bow down only if the motor is trimmed down as well. Foils are generally added to avoid porpoising and to aid hole shot. The secondary effect (when purposely trimmed down) is to drive the bow down.. When the bow is plowing (or when the boat has more wetted surface) the same twisting force is present as you had before. One force holds you back (bow down) while another stresses the transom with the bow up. When you take the measurement -- if you really want to get into that amount of investigation, depends on how you have the motor trim set at the time.

There is no free lunch anywhere. The discussion indicated nothing of the kind. Nowhere did I say anything like this. If one put a "big enough" foil on the engine the force would be greater than if there was none. If you read carefully, you'll see that I said that trim is even more critical with a foil than without one.

Balance is the key, as it is in most things. Too much one way or the other only makes things worse. I learned in high school physics that force vectors can combine to increase the net force, or to decrease it. Two forces of equal strength acting in opposing directions balance each other out, producing a net effect of zero. Push on a wall with enough force, and it falls down. Push with that same force, but counter it with a force in the opposite direction, and it stays up.

Perfect balance in this case is not desirable. You want the bow slightly out of the water when below planing speeds. At the same time, you don't (no, amend that. *I* don't. I have no idea what "you" want.) want the bow so high that you lose control and can't see what's in front of you.

Admittedly, it would be better if I were to counter the 300 lbs. combined weight of me and the motor with additional weight in the bow, but that just isn't practical in my case. I can only do so much. Once the boat is planing, the bow comes down and stress on the transom is reduced because there is less drag from the water. The critical point is slightly sub-planing speed, and that's where I maintain that the foil reduces net stress rather than increases it.

 

[Gary H NC]

Re: Will a "hydrofin" reduce transom strain?

Makes me wonder just how thick is the transom? Did you use new plywood and go all the way to the bottom?
I would get rid of the 2x4 and add a piece of aluminum angle or u channel...
I see people who use just a small square of plywood for the center of the transom when it should span farther across to distribute the force of the engine pushing the boat.
Hard to say though not seeing what you are working with.

 

[tjandrews]

Re: Will a "hydrofin" reduce transom strain?

Makes me wonder just how thick is the transom? Did you use new plywood and go all the way to the bottom?
I would get rid of the 2x4 and add a piece of aluminum angle or u channel...
I see people who use just a small square of plywood for the center of the transom when it should span farther across to distribute the force of the engine pushing the boat.
Hard to say though not seeing what you are working with.

One inch thick on the inside, all the way across and extending down about eight inches - just over half way. The original board was 1 inch plywood, but my replacement is one inch rough-sawn white oak, new. There is also a 12" x 8" rectangle of 5/8" plywood on the back where the motor mounts.

The problem with the transom actually comes from the sides of the boat flexing inward at about a foot from the back. The 2 x 4 is mounted as a "shelf" at the bottom of the white oak board, and is tight enough to the sides to reduce that flexing. The flexing hasn't been eliminated, but I have the feeling that it really shouldn't be. Originally there was a 1 x 2 in that same position, but it was poorly fastened in the original design.

This was a cheap boat, not nearly as well-made as it should have been. Had my father not made the improvements he did, it would have been long gone by now.

 

[tjandrews]

Re: Will a "hydrofin" reduce transom strain?

Makes me wonder just how thick is the transom? Did you use new plywood and go all the way to the bottom?
I would get rid of the 2x4 and add a piece of aluminum angle or u channel...
I see people who use just a small square of plywood for the center of the transom when it should span farther across to distribute the force of the engine pushing the boat.
Hard to say though not seeing what you are working with.

Here are a couple of photos of the boat as it is at this moment. Please ignore the paint/finish work that needs to be done yet. I'll get to that as I get time.

 

[mickjetblue]

Re: Will a "hydrofin" reduce transom strain?

JB - I have used the junior fin on an 18hp motor, and I believe that it resulted in better
control and planing, overall. I could also sense that different amounts of "strain" were
taking place on the transom at different times than without the fin. All the same, 15hp
is still 15hp, and that is much more than 10hp.

The biggest unknown, in consideration of all the variables, is how much damage was done
to the transom from the tree falling on it, and it being pounded back to shape. The 2 x 4
addition was ok for the 10hp, for awhile, but I would not feel comfortable with this setup and a 15hp motor.

I would suggest adding a heavy duty piece of aluminum angle across the top of the 2 x 4.
The boat is rated at 17hp, but the transom having been damaged creates questions.
I would also add a piece of 1/4" aluminum plate on the inside of the transom from the top
down to the new aluminum angle. But, I am no maritime engineer, either.

Good luck!

 

[tjandrews]

Re: Will a "hydrofin" reduce transom strain?

JB - I have used the junior fin on an 18hp motor, and I believe that it resulted in better
control and planing, overall. I could also sense that different amounts of "strain" were
taking place on the transom at different times than without the fin. All the same, 15hp
is still 15hp, and that is much more than 10hp.

The biggest unknown, in consideration of all the variables, is how much damage was done
to the transom from the tree falling on it, and it being pounded back to shape. The 2 x 4
addition was ok for the 10hp, for awhile, but I would not feel comfortable with this setup and a 15hp motor.

I wasn't clear about the tree. We were at a camp on the St. Lawrence River in the late 60's, and there was a storm coming overnight. The boat would have been better off in the water, but at the time we didn't think there was room for it to bounce around undamaged. So, we pulled the boat out of the water, turned it upside down, and tied it to the tree so it wouldn't blow away. The storm that night was particularly violent, and when we got up the next day four trees along the shore had come down, including the one we had tied our boat to. The tree didn't fall on the transom itself. It fell across the boat at the second seat from the back, 4.5 or 5 feet from the transom. The boat was flattened at that point, but there was no visible damage to the transom. Many of the rivets were strained at least enough to leak, so the ones fastening the transom onto the sides are probably no exception. Fortunately, there were no tears in the aluminum.

We had to put the boat on the trailer upside down and backwards to get it home - which garnered a lot of strange looks on the interstate. When we got back, Mom took it to a repair shop for an estimate. They told her they could probably repair it, but it would cost her more than the boat would be worth afterwards. She asked the cost - they said $150. (Remember, this was the late 60's.) She said to them, "You mean you have a boat this size that you'll sell to me for $150?" They said, "Welllll - no." She said, "Fix the boat."

I was just a kid, and I wasn't around to see the process myself, but from what I was told they removed the seats, then used some kind of press to re-form the boat. After some touch-up hammer work, the seats were put back in, and they tested for leaks by filling the inside of the boat with water. Nearly every rivet leaked. Their solution was to paint the hull with something they called "special sealer paint." (The boat had been unpainted before this.)

That did stop the leaks, and Mom had her boat back. Some of the paint scraped off now and then, and as leaks showed up we sealed trouble spots with the clear liquid fiberglass stuff that's usually used with the cloth, and it had to be repainted several times. Mom liked blue, so blue it was - sky-blue if we could find it. The fiberglass stuff wouldn't hold more than a few years, because expansion and contraction would eventually break it loose, but we simply re-applied, and it was fine.

Mom owned that boat until she died, the end of June. Caught more perch, rock bass, smallmouths, walleyes, and northerns from her seat (second from the front) than I can count. She never regretted having the boat fixed, or spending the money to keep it usable. I don't blame her. I think she got her money's worth several times over.

At any rate, the main reason the motor might pull back under full power isn't so much a weakness in the transom itself. The problem comes from the sides of the boat up where the tree landed. When you pull back on the transom, if you can get where you can pull hard enough, the sides of the boat flex inward, thus lengthening them and allowing the transom to tilt back slightly. I suspect they didn't get the curve of the sides quite where it was before the tree, and it wouldn't surprise me if the metal had been stretched a bit at that point. The old plywood had gone bad due to insects and rot, and that certainly didn't help any, either.

The new white oak board is stronger than the old plywood had been for years. I seriously doubt that there will be any problem for a good, long time with the transom itself bowing - even under the strain of a 15HP motor. Since replacing the old, original board, the flexing of the sides has been reduced because of two things. First, the corner brackets across the top had still been fastened to the old board using rivets - which had loosened up over the years. I replaced those rivets with quarter-inch bolts, and that was a BIG help. Second, the 2 x 4 is not only fastened to the transom - it's also fastened to the sides using three 2" lag screws on each side. That was a big help too, though not as much as tightening the corner brackets. BTW, my father added his 2 x 4 in there about 30 years ago, so yeah, it was OK- for a while.

I don't think there will be any problem handling the 15HP motor, as long as things are kept in good shape. I just wondered if the hydrofoil could help, too. I didn't realize what a sore subject it seems to be.