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Question on how far the prop should be bellow the transom?
- Thread starter whynot
- Start date
When I was building boats I always built the motorstand so the prop was the length of a soda can off the runners in the bottom of the boat and the length of that same soda can off the transom. We built open fiberglass boats with low sides and auto engines and the deep prop cut allowed the prop to bail water if ya took some over the transom.
Newer higher sided boats don't necessarily need this so lots are setting them just below the top of the transom. I have seen really short propped boats with them all the way above the transom. You get a bit more push up high instead of in the bottom of the boat too.
Remember the higher the prop the greater the torque effect on the boat also, so you may want to set things up for some compromise between best push and less torque effect. Or if you are a pureist, go for the gold completely above the transom.
There is no reason you can't set the tips lower as you asked but as you see there are considerations to be aware of in the choices you make.
Hope this helps a bit.
Scotty
Newer higher sided boats don't necessarily need this so lots are setting them just below the top of the transom. I have seen really short propped boats with them all the way above the transom. You get a bit more push up high instead of in the bottom of the boat too.
Remember the higher the prop the greater the torque effect on the boat also, so you may want to set things up for some compromise between best push and less torque effect. Or if you are a pureist, go for the gold completely above the transom.
There is no reason you can't set the tips lower as you asked but as you see there are considerations to be aware of in the choices you make.
Hope this helps a bit.
Scotty
deepsea diver
Well-known member
this is a good question, and people will have many "OPINIONS".
The way I see it , None would be better, but I have seen alot of boats with 3 to 4 inches of prop inside the transom. They do Okay, ride rather well. A while back, I think "HGM"s dad had some new metal work done and the Prop went way to deep into the boat. I believe he said the boat did not handle to well.
Look at a full deck boat. one with a 9 to 12 inch transom. The motor is set low and the thrust pushes the the hull across the water and ground. Close to Parallel.
Now look at a boat with 23 inches in the stern, If the motor is set high for the big prop and the prop tips clear the transom. You most likely are going to feel torque roll, but the driving force3 may push the bow down and the boat may want to plow on the nose.
But if you set the prop do deep ,then the back inside of transom will act like a wind sock. You have alot of wind blowing the stern of the boat backwards, and the top of the prop pushing the bow forward and down.
You will just have to make adjustment to get it the best you can.
This is why I personally like the full deck boats, You can keep the engine lower, Which gives you better action with the running surface. You don't have to worry about things coming into the boat, water and grass. It's easy to clean.
What is the magic distant? It all depends on the entire boat and the weather that day.
Oh, No offense to you tub boat guys. Just Chillen and Killen some time.
The way I see it , None would be better, but I have seen alot of boats with 3 to 4 inches of prop inside the transom. They do Okay, ride rather well. A while back, I think "HGM"s dad had some new metal work done and the Prop went way to deep into the boat. I believe he said the boat did not handle to well.
Look at a full deck boat. one with a 9 to 12 inch transom. The motor is set low and the thrust pushes the the hull across the water and ground. Close to Parallel.
Now look at a boat with 23 inches in the stern, If the motor is set high for the big prop and the prop tips clear the transom. You most likely are going to feel torque roll, but the driving force3 may push the bow down and the boat may want to plow on the nose.
But if you set the prop do deep ,then the back inside of transom will act like a wind sock. You have alot of wind blowing the stern of the boat backwards, and the top of the prop pushing the bow forward and down.
You will just have to make adjustment to get it the best you can.
This is why I personally like the full deck boats, You can keep the engine lower, Which gives you better action with the running surface. You don't have to worry about things coming into the boat, water and grass. It's easy to clean.
What is the magic distant? It all depends on the entire boat and the weather that day.
Oh, No offense to you tub boat guys. Just Chillen and Killen some time.
I have a 14x7.6 combee with a big block and a 2.38 rotator with a 78 inch 3 blade, The prop is about 1 inch below the transome and if I didn`t have a splash well back there i would lower it,the torque roll is terrible.Bad enough when coming out of three forks heading to helen blazes if i stab it as I`m turning right into the canal the boat will lean out to the left :shock: You will be just fine with that much prop below the transom.Let us know how it handles,that small block shouldn`t roll as bad but drive carefully the first run
At 13 foot your boat should go like hell :twisted: and run the hill with ease
Ron Miller
Well-known member
Interesting reading. 12' hull -- well you will need the professional help of the southern boys because a small hull up here in Mi. is a 14' .. We run a 15' x 8 with a Stinger gear and 600hp and have no tork role but I am assuming that is because the prop and engine are turning different directions.. As for the prop below the transom I done a air flow test using yarn -- tied on over 100 pieces all over the cage -gas tank -- wiring --- any place I could get it on there.. then I set the throttle at 1000 rpm took pictures and video of it and then at 2000 and 3000 rpm.
Our prop is 31/2 " below the transom and all of the air from 1" above the transom was blowing into the hull forwards. I showed this to two aviation engineers and they agreed there was a huge thrust loss from the prop being below the transom .. Plus it also greatly increases the sound coming from the prop.. To change this on our hull and I just have not had the time yet.. I want to cut the transom down to ?2" below the prop and then rebuild it on a angel to the back and back up to the original height. I know that other hulls are built this way and I do believe that they gain more thrust with less NOISE and the sloped back transom should help push the hull up when you stop fast.
Good luck with what ever way you go.
Our prop is 31/2 " below the transom and all of the air from 1" above the transom was blowing into the hull forwards. I showed this to two aviation engineers and they agreed there was a huge thrust loss from the prop being below the transom .. Plus it also greatly increases the sound coming from the prop.. To change this on our hull and I just have not had the time yet.. I want to cut the transom down to ?2" below the prop and then rebuild it on a angel to the back and back up to the original height. I know that other hulls are built this way and I do believe that they gain more thrust with less NOISE and the sloped back transom should help push the hull up when you stop fast.
Good luck with what ever way you go.
whynot
Well-known member
Ron thanks, that was a very interesting way to find out but also very informative. After hearing that I will consider raising my engine until the prop is completely out of the transom and then see if I can live with the torque roll, if not maybe i will cut my blades down some.
Ron Miller
Well-known member
Check with those who have the boats like yours .. As I said I have no experience with your size of hulls and I can imagine that the torque roll could be dangerous espically if you have short sides.
whynot
Well-known member
Yes I understand torque roll and all that good stuff, the hull is a 13-6/7-3 cotton mouth. This will be my 3rd boat and I am just trying to figure out where to put the prop in order to get the most out of it without having to worry about the torque. My last boat had the same setup but it was a small boat for it so I put the prop in the boat pretty far so not to sink it with the torque. Trying to figure out the most common placement for the prop in relation to the top of the transom. Always looking for new ideas, appreciate all that I have gotten so far and keep them coming.
Ah ha, so your not new to airboats, then you "should" be able to deal with the torque roll, especially since it is not a round chimed boat.
I don't run long props on any of MY boats. All the big stuff I ran did have long props. I stick to under 72" for myself and learn to run the boat with whatever thrust it makes.
My new 11' boat has a 66" prop but is a 2/3 decked hull with round sides.
The flip side is that if you make it clear the transom, it is always easier to chop a bit off the bottom of a motorstand than to add to one. You can always cut it down some if you don't like it.
This is a worthwhile conversation but in the last analysis your going to have to set it up one way and see if you like that or not. You can always change it if you don't.
At least you bothered to ask, and that puts you way ahead of some builders.
Go for the gold man.
Scotty
I don't run long props on any of MY boats. All the big stuff I ran did have long props. I stick to under 72" for myself and learn to run the boat with whatever thrust it makes.
My new 11' boat has a 66" prop but is a 2/3 decked hull with round sides.
The flip side is that if you make it clear the transom, it is always easier to chop a bit off the bottom of a motorstand than to add to one. You can always cut it down some if you don't like it.
This is a worthwhile conversation but in the last analysis your going to have to set it up one way and see if you like that or not. You can always change it if you don't.
At least you bothered to ask, and that puts you way ahead of some builders.
Go for the gold man.
Scotty
