freshwaterhunter I was researching the same question a while back and made a document with some data I will share.
Transom data from different SA posts: :salute:
Mine I believe mine is 9 inches in the boat. I'm running an R series.
a lower push point will gain you more than any so called loss of thrust.... and by the way thrust is a byproduct of what the prop really does, and that is screw through the air... it’s the same as an out board motor prop in the water .... All you young guys that have never ran a wood prop, here’s how they used to gage a prop, a 72 -32 wood prop in theory would move 32 inches forward every time it made 1 revolution, the best thing you can do to help your airboat is to have un obstructive air feeding your prop no matter what kind it is.... plus the lower the center of push of your boat the less down force you put on the nose, I have my 70 in prop 10 inches below my transom.
I've built and ran a lot of different airboats from ride to race boats, I can say that my best handling and performance boats all had between 10 to 12 inches inside the transom, handling and lower point of thrust far out ways any down side my boat right now has a 81 inch Sensenich 12 inches inside and it works.
Have mine 11 inches in , 82" R . The tips draw air in and middle of prop is where thrust generates. That’s the purpose of the winglets to cup air and force it toward middle and not escape off end.
11'' deep on a 16x8 marsh master. Works great.
you should not have any noticeable loss in thrust, with only 7" below the transom. I have a boat with almost 12" below!!! And no considerable loss of thrust there. ( I have been told ) The boat has a 84" 3 blade R and is 20' + HEAVY, but will run dry loaded with 7 passengers!
i run mine about 9'' in the boat and 4'' from the stringers i didn’t lose any thing that matters and i dont need a pump the prop bails it just fine 82'' super wide..
The lower the better!!! Don’t worry about the prop being down below transom. The end of all props flatten out under RPM and give no push at all. Push comes from the hub out where all the pitch is. I will say again, simple test. Take an empty soda/beer can and put it on a flat surface. Take a finger and place it one inch from top and push it across table. Now do the same thing with your finger one inch from the bottom. Make cents? Anywhere below center is good, anywhere above center not so good when metal work is being built the most important performance thing is the height and location of the stand. If you can just slide a pencil between the end of prop and stringer, nail it. On a deck over between the prop and deck. Just one old man’s big "O". Former member "NARA", rider, builder, tear um up, fixer since 1968. J Renney (ps all engine stands should be built with an up and down adjustable nose mount)
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.
There is a direct correlation between downward force on the bow and engine height. Also when the motor is higher the boat will torque roll more. Due to higher horsepower levels car motor gearbox boats are the most sensitive to engine height. Two or three inches will make a very noticeable difference. With the carbon fiber props on a ride boat I keep the tip’s 1½ inch off the bottom on race stuff I try for 3/8 of an inch. Of course the lighter and smaller a boat is the more noticeable these changes are. So if you’re running a 16ft tour boat this height will not matter as much as a 13ft ride boat. The higher the motor the harder it is to keep them from riding on the nose and the more the boat will torque roll. Sometimes losing a 100lbs of thrust will drastically improve how a boat rides! You will not feel this so much on boats over 2,300lbs or over 16ft long but you build a light medium size airboat and those 6 inches of height will COMPLETELY change how loose and how well the boat rides!
