Shaft length for TM

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bobberboy

bobberboy

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Can anyone tell me how to figure the correct length for a trolling motor shaft? My TM came with my used boat and seems really too long. Is there a correct depth below the water surface or how much adjustment should I be able to make? When the prop is about 6"-8" below the surface there is still about 20"-24" of shaft above the deck. It seems way too long and it is in the way, plus it takes up a lot of deck space.
 
From Cabela's, hope it helps.

For a transom (rear) mounted trolling motor location, measure from the top of the transom where the motor will be clamped to the water line. By adding 20 inches to this measurement, you will have the recommended size. For example, if the distance from the top of the transom to the water line is 15 inches, adding 20 inches gives a shaft length of 35 inches. Usual shaft sizes are 30, 36 and 42, so in this example, the 36 inch shaft would be the recommended choice.

Transom Mounted Trolling Motors

Measurement* Recommended Size

5-10 inches 30 inches

10-16 inches 36 inches

16-22 inches 42 inches

To mount the motor on the bow (front) of the boat, measure from the mounting surface to the water line. For distances up to 22 inches, add 20 to obtain the recommneded shaft length. For measurements of 22 to 28 inches, the recommended shaft length is 45 to 52 inches. For measurements of 28 to 34 inches, a shaft length of 54 to 62 inches is recommended. It is better to have the shaft slightly longer than too short; the shaft can be slid up or down in the mount for adjustment. If the shaft is too short, the motor will be lifted out of the water as the bow encounters waves and the prop will over-rev, splashing and making noise.

Bow Mounted Trolling Motors

Measurement* Recommended Size

10-16 inches 36 inches

16-22 inches 42 inches

22-28 inches 45-52 inches

28-34 inches 54-62 inches

*Measurement is made down from the mounting surface to the water line.
Click to expand...
 
I've always been told that the right depth to have a trolling motor in the water is just below the level where it slurps and you can see a vortex in the water. Where the head sits above that seem to make little differance as long as it's in a functional position i.e. you can reach it comfortabley. If it works, don't fix it.
 
tccanoe said:
I've always been told that the right depth to have a trolling motor in the water is just below the level where it slurps and you can see a vortex in the water. Where the head sits above that seem to make little differance as long as it's in a functional position i.e. you can reach it comfortabley. If it works, don't fix it.
Click to expand...

It's just that there seems to be so much shaft sticking above the deck. The guy I bought the boat from was a bit of a Craig's List jockey and I think he got a deal on this TM. I don't think it was sized for the boat from the start. I am going to measure next time I'm on the water to at least get an idea of what I'm dealing with. It can't be too critical - when I lean over the bow to see and adjust the height the boat tips down in front and as soon as I step to the back the motor raises up in the water.
 
Couple of thoughts: I assume the TM is bow mounted. Can you put it on constiant run and move toward the back of your boat? When that baby goes to slurping you'll know it. I think a little deep is better than a little shallow. Next, can your remove the head and trim off some of the shaft? (boy, that sound scarry and exciting Let's do it!) No really. If the wires in the head don't plug into each other then clip them and add electrical joints. Advice is cheep and it's easy to spend other people's money. TC
 
tccanoe said:
Couple of thoughts: I assume the TM is bow mounted. Can you put it on constiant run and move toward the back of your boat? When that baby goes to slurping you'll know it. I think a little deep is better than a little shallow. Next, can your remove the head and trim off some of the shaft? (boy, that sound scarry and exciting Let's do it!) No really. If the wires in the head don't plug into each other then clip them and add electrical joints. Advice is cheep and it's easy to spend other people's money. TC
Click to expand...

First, and part of the problem is, yes, I can get access to inside of the head of the TM - I have already partially removed it by hitting the dock. That's one reason why I want to shorten the shaft. As the motor sits now, in order to fit on the factory mounting plate and still get clearance for the motor, I had to mount the TM so the head goes beyond the side of the boat when not in use. A shorter shaft would get the head inside the boat. I have not looked yet at whether I can shorten the shaft but that would be my goal. I'm not worried about cutting the wires - I do stuff like that all the time. I would like to make the shaft the shortest I can and not compromise how the TM works. I still haven't been able to measure the distance to the waterline but that's the next step. If I end up changing the TM, I'll post pics. Thanks everybody.
 
BB, I would never question your ability to tackle your trolling motor, top or bottom. Set it to the depth you think is right, put it on constiant run, move to the middle of the boat. If it don't gurgle it's probably right. Trim to fit. When you're in the boat by yourself and you move all the way back, any motor will prob gurgle. (I use the word gurgle alot). I have no doubt you'll get it the way you want it. When we answer these questions in a basic way, a new-commer may get an answer/idea that will help them along.
 
tccanoe said:
BB, I would never question your ability to tackle your trolling motor, top or bottom. Set it to the depth you think is right, put it on constiant run, move to the middle of the boat. If it don't gurgle it's probably right. Trim to fit. When you're in the boat by yourself and you move all the way back, any motor will prob gurgle. (I use the word gurgle alot). I have no doubt you'll get it the way you want it. When we answer these questions in a basic way, a new-commer may get an answer/idea that will help them along.
Click to expand...

Sorry man, I hope I didn't sound terse. It's just that the first time I put the TM on the boat I was also getting used to my new outboard. Well, forward being reverse and vice-versa, I managed to run the boat and TM into the dock and cracked the head of the TM. So intentional or not, I do have access to the head of the TM. I also am kind of notorious for not being afraid to take stuff apart - sometimes before I weigh all the consequences. That coupled with a bad memory or early-onset Alzheimers (or both!) and things don't always look the same out the other end of the project. Let's just say that you're glad I'm not your surgeon. Anyway, this is the first bow-mount TM for me and like I said earlier, I don't think it was sized specifically for my boat. I'm just trying to work through the options. I also find a little frustrating that the factory sometimes does things in strange ways. My rear seat mount was directly in front of the motor where it's impossible to run the boat without being a contortionist. The location of the TM mount was such that I couldn't avoid having it stick beyond the side of the boat. I like to do the work on the boat but not redoing work that could have been thought out better. Bottom line - I've got a cracked-up brand new TM and I want to keep it from happening again. Sorry for all the wind...
 
Ok, so I finally took a few measurements. The TM shaft is 52" as I suspected. My boat has 20" sides. According to the chart devilmutt posted (thanks!) my shaft length should be 42" (For distances up to 22 inches, add 20 to obtain the recommended shaft length.) There is one small bolt holding the shaft to the head - certainly not enough to keep me from cutting the shaft down. Also, as I now have a clear view of the interior of the head, there's probably room in there just to coil up the extra wire. I'll let you know.
 
I cut the shaft down today. It was easy and there's no reinforcement where the screw goes through. I don't know what the shaft is made of, something like fiberglass but it cut easily and cleanly with a hack saw. Drilled new holes through the shaft, soldered the wires back together and it was done. To keep the string of bad TM luck going though, I managed to let the motor drop off the bench and broke the prop. So $15 later I'm ready to try it out. It's been raining for about 40 days and 40 nights so I hope tomorrow will be the day to try it out. Actually, unless there's lightning, I'm going out tomorrow.
 

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