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prop height?

R

railroad_foreman69

Member
hey fellas, was wondering why the prop has to be in the boat? why can't ya put it out of the boat behind the transom? more room in boat, more distance from the prop, and lower center of gravity? i'm new at this airboat thing, so please be gentle ?
 
you have to be able to blow the back wash out of the back of the boat you need the prop 2 to 5 in inside the transome
 
I was looking at some tour boats out at Holiday Park last night and noticed they all had the props beyond the transoms, over the water. I was thinking this may be for reduced noise for the passengers on them. On a normal ride boat I would be afraid of the prop hitting something when pulling off of dry ground or running over debris. I have always placed the prop even with the transom on open boats, or about 1-1 1/2 in. over the deck on a decked in hull.

Larry
 
Railroad - I would be worried about something hitting the props, especially if crossing dikes, logs, or levees. When the transom drops off the ledge, the turning prop could hit as the boat slides into the water. It would also push the rudders further back.

About a decade to two ago, there were lots of single rudders that looked like cross-sections of airplane wings. One of the biggest reasons I heard they got away from them was that they would hit levees while crossing them.

Also when running in deeper water and you let off the gas quickly, your wake will catch up and sometimes spill over the transom. With the prop behind the transom, you would have the prop regularly hitting the water and that is not a good situation. Several good prop strikes into water with a wooden prop and it can start having some dangerous issues. Composites are stronger, but hitting water is still much more stress than what they are designed for. I would be scared of scattering a blade.
 
The general rule is dip the prop tip 3" from the top of transom and 3" forward (inside) away from the transom. Rigging your prop out past the transom would be inviting all kinds of opportunity for trouble. Of course there is the chance that when your prop blows apart it won't go through the bottom of your hull.
 
The number one reason to run your prop down in your hull is to lower your CG. In a car motor this makes a huge difference it’s not as critical on an aircraft because they run such a small prop. If you have an 80 inch prop 2 inches of your deck on a 15 sided hull the center of your prop is 57 inches off your bottom. This causes torque roll and plowing also the higher the motor sits the more it pushes down on the nose. If you run an 80 prop 2 inches off the bottom the props center will only be 42 inches from the bottom. That 15 inches makes a huge difference in how the boat performs. Most aircrafts run a 68 to 72 inch prop so it’s nowhere near as critical but it still helps.
 
So on an aircraft engine with a 72" prop will I probably be ok if the prop tip extends right a 5" below the transom?
 
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