How far should PROP extend below the top of the transom?

Big Casino · **Posted:** Mon Mar 29, 2010 5:47 pm

Help me out with this one ....

Is there a rule of thumb or some other specific info on how much blade (length) should extend below the top of the transom inside the hull?

I know you need some, but too much and you loose thrust. Not enough and, well I dont know ... ?

What do ya'll think?

Lakerunner · **Posted:** Mon Mar 29, 2010 5:56 pm

I don't know either what the gurus think. BUT, me and my buddys all use the same scale or tape.. We put the tip of he blade the lenght of a bud light can inside the hull. Maybe someone on here has done some testing and have a real number, but this works for us.

MYDIXIEWRECKED · **Posted:** Mon Mar 29, 2010 6:18 pm

I build all of mine 5 inches inside. so a beer can is about right.(even thought I haven't measured a beer can!) :lol:

machinestles · **Posted:** Mon Mar 29, 2010 6:18 pm

I see the beer can done mostly. So many folks wouldnt do it it it wasent a good way to do it. I will say this though, my boat was setup to run a 78 in prop set about a can in and below. I now have an 82 in 3 blade R on and its a good can and a half below the transom. I tried a friends 79 inch R blades before I bought mine and the only difference I noticed between the 79 and 82 is about 50 to just under 100 rpm difference in cruise rpm at the same pitch setting. The added distance below the transome does not seem to be a problem on my boat.
lester

Lakerunner · **Posted:** Mon Mar 29, 2010 6:23 pm

MYDIXIEWRECKED wrote:

I build all of mine 5 inches inside. so a beer can is about right.(even thought I haven't measured a beer can!) :lol:

Chester your close enough.....Budlight can is 4 13/16" The extra your putting in might not hurt!! :lol:

harley · **Posted:** Mon Mar 29, 2010 7:11 pm

I set mine at , 5 - 7'' , below transom.. they seem to work fine at that setting...

swampdragster · **Posted:** Mon Mar 29, 2010 7:45 pm

i run mine about 9'' in the boat and 4'' from the stringers i did'nt lose any thing that matters :lol:

and i dont need a pump the prop bails it just fine :wink:

82'' super wide..

TSnake · **Joined:** Wed Aug 16, 2006 11:00 pm **Posts:** 145

I had to go check my boat. Sure enough, one beer can below the transom. Of course I had to empty that can first to ensure an accurate measurement. Now the question that's bothering me – why? What benefit does this give compared to the prop not extending below the transom? It seems like you would be losing a little thrust because of the air hitting the transom instead of pushing the boat along plus that mess of bug guts I've always got on the inside of the transom. I have to grab another can and go verify that measurement now.

Lakerunner · **Posted:** Mon Mar 29, 2010 8:04 pm

swampdragster wrote:

i run mine about 9'' in the boat and 4'' from the stringers i did'nt lose any thing that matters :lol:

and i dont need a pump the prop bails it just fine :wink:

82'' super wide..

Funny thing happened...I took a 0-320 off my boat and installed a 0-540. The distance to the bottom is the same and the distance from the rudders are the same...I even used the same bolt holes in the stringers and now it want bail my boat out as good as it did before. Before it would bail the boat out while sitting still on dry ground, now I have to take off before it starts bailing.

JAMES · **Posted:** Mon Mar 29, 2010 8:24 pm

i run all my boats a can down a can from the transom always been good my buddys new dd is only 2" down :shock:

seems to do ok for a big heavy cotton mouth 14fter 350 dd been that way ssince the boat was built :shock:

CJB0470 · **Posted:** Mon Mar 29, 2010 8:32 pm

What is the benefit of having the prop below the transom? I have a 13.6 Cottonmouth with a 220 swinging a 72" Stump Puller and it is all the way out of the hull. It seems you would loose some top end or does this make it more stable, lower center of gravity and all...

nomad41 · **Posted:** Mon Mar 29, 2010 8:48 pm

I was told it helps keep inside clean.If you have a little water in boat.It will suck it out.

tonyld67 · **Posted:** Mon Mar 29, 2010 9:07 pm

Guys Iv'e wondered the same thing,i had a guy build me a engine stand last yr & I'm runnin a ww70in 0320 & my prop is all the way out of the back & the way the stand is i can't lower it to get it inside the transom,don't know if this benefits me being outside the transom or not,seems to run fine but iv'e always wondered the same thing,every boat iv'e ever seen is inside the transom!!

dantheairboatman · **Posted:** Mon Mar 29, 2010 9:09 pm

Think of it as a cost benefits. Observe from a safe distance behind your boat how far in on your prop you go before you feel the air. The prop gathers the air into a collum. picture it shaped like a funnel. then consider the benefits of having a lower point of push and center of gravity.
Also consider the radiator or oil cooler in front of the prop creating dirty air. if your prop isn't blowing much air 6to 10 inches from the tips and you have dead space staticly in front of the prop from rad/oil cooler/gastank. then it is a gain to be low in the boat and not push down on the nose and not be top heavy. when the boat is moving the air running down the bottom of the hull along the stringers is being then forced to the prop b/c of transom. look at how high the engines are in race boats. we will soon do some testing to show this. we do the coke can tradition and were not sure why? i think there are several things like this on boats i would like to get to the bottom of!

Lonewolf9029 · **Posted:** Mon Mar 29, 2010 9:22 pm

On my build I was told by the man I bought my stainless from; and thats all they do there is build airboats, it's not so much the can thing-You want that prop to be right in the center of the 2 hoops. And after I pushed the engine stand back, to line up the center of the hoop holes, it came out to 5-6" of the transom. I built the height to 6" below the transom. So the can thing is a good "rule of thumb". Don

Champ18ft · **Posted:** Mon Mar 29, 2010 9:25 pm

Maybe Dagwood from Sensenich will chime in here and let us all know how and where a prop gets its air and push. I was always under the assumption that a prop brought it air in from the sides. That the center of the prop gave the push and the tips gave the speed. It would be nice to know the real truth.

dantheairboatman · **Posted:** Mon Mar 29, 2010 9:46 pm

we can set off some smoke bombs in front of the boat on the smart trailer and see whats going on :bom:

I don't wan t to come across as a know it all. my understanding is that it pulls it in the front at a 45deg and gathers it into a the center from the cup of the blade. i would like to here it beter exsplained by them as well.i bet the spell better too? :idea:

Waterthunder · **Posted:** Mon Mar 29, 2010 9:47 pm

Some times you gain things by loosing thrust! On some boats I would never put the prop in the transom. Other boats it's best to put the prop below the transom. Case in point If you have a big or heavy boat put ALL of the prop out of the transom!

Champ18ft · **Posted:** Mon Mar 29, 2010 10:12 pm

Waterthunder wrote:

Some times you gain things by loosing thrust! On some boats I would never put the prop in the transom. Other boats it's best to put the prop below the transom. Case in point If you have a big or heavy boat put ALL of the prop out of the transom!

OK My question is then this. If you are running a big Prop wouldn't that make it top heavy. In a slow turn that would be nasty.

DAGWOOD0520 · **Posted:** Tue Mar 30, 2010 5:34 am

I do believe that it has more to do with how everything is setup. If you are building alot of power on a short boat i would think to keep the motor lower so it doesn't try to push over on the nose as bad, but if the same power is on a much bigger setup you would be able to keep it higher and not cause the rig to act funny.
i am sure there is a method to the madness, but I have seen stuff all over the place.

air-roberts · **Posted:** Tue Mar 30, 2010 6:43 am

i allways set mine 3" below like dave said you go more and you might have more rpm but lose thrust play with a home fan thats how i learned alot you can see the differant rpms and thrust lose

Southern Style · **Posted:** Tue Mar 30, 2010 8:01 am

Thats all I have ever heard was the "beer can" theory, and then the "cleans out your hull" theory, but never understood.. Have seen them OUT of the transom and some JUST INSIDE the transom.. Mine is 5 inches inside the transom and was told that it is more the center of the prop that gives you your thrust. That is why the ends of the prop do not have that RADICAL twist at the ends. That is there for balance and to help the speed of the prop, so the ends being inside the hull are really not a big deal, they still spin and help..........

Does that make sence???????????????????????

watersports7 · **Posted:** Tue Mar 30, 2010 1:56 pm

great topic, air travels down the blade several inches (from the tip) before it starts blowing out
(Bobby Hoffman, Basic Airboat Setup Tips, Airboating Magazine, March/April 2009)

leo_081103 · **Posted:** Tue Mar 30, 2010 3:41 pm

dantheairboatman wrote:

Think of it as a cost benefits. Observe from a safe distance behind your boat how far in on your prop you go before you feel the air. The prop gathers the air into a collum. picture it shaped like a funnel. then consider the benefits of having a lower point of push and center of gravity.
Also consider the radiator or oil cooler in front of the prop creating dirty air. if your prop isn't blowing much air 6to 10 inches from the tips and you have dead space staticly in front of the prop from rad/oil cooler/gastank. then it is a gain to be low in the boat and not push down on the nose and not be top heavy. when the boat is moving the air running down the bottom of the hull along the stringers is being then forced to the prop b/c of transom. look at how high the engines are in race boats. we will soon do some testing to show this. we do the coke can tradition and were not sure why? i think there are several things like this on boats i would like to get to the bottom of!

EXACTLY!!! again, "picture it shaped like a funnel"

swampdragster · **Posted:** Tue Mar 30, 2010 8:27 pm

it's ok to be a know it all , that way we know were to go for the right ansers :wink:

rock on Dantheairboatman

How far should PROP extend below the top of the transom?

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