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

Automotive powered airboat discussion.
User avatar
Big Casino
Site Supporter - IV
Site Supporter - IV
Posts: 1149
Joined: Wed Nov 11, 2009 7:03 pm
Location: Suwannee County

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

Postby Big Casino » Mon Mar 29, 2010 6:47 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?
"...This above all: to thine own self be true..."

User avatar
Lakerunner
Site Supporter - IV
Site Supporter - IV
Posts: 1138
Joined: Tue Oct 21, 2008 11:13 am
Location: Maurepas, La
Contact:

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

Postby Lakerunner » Mon Mar 29, 2010 6:56 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.
"COONASS COUNTRY"

12'x6' Aluminum /0-540 A/V
"Marines - Providing Enemies of America an Opportunity To Die For their Country Since 1775"
USMC 1964-1970

User avatar
MYDIXIEWRECKED
Site Supporter - IV
Site Supporter - IV
Posts: 3633
Joined: Tue Dec 18, 2007 7:48 am
Location: Plantation,fl Area 3A south camp 9 1/2

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

Postby MYDIXIEWRECKED » Mon Mar 29, 2010 7:18 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:
THOUGHT FOR THE DAY
"People sleep peaceably in their beds only because rough men stand ready to do violence ON THEIR BEHALF"

Lifetime member NRA
Owner camp 9 1/2 in beautiful 3A south, Everglades

machinestles
Southern Airboat Member
Posts: 192
Joined: Tue Feb 17, 2009 8:35 pm
Location: Dunnellon

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

Postby machinestles » Mon Mar 29, 2010 7:18 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
Old school american manufacturer.

User avatar
Lakerunner
Site Supporter - IV
Site Supporter - IV
Posts: 1138
Joined: Tue Oct 21, 2008 11:13 am
Location: Maurepas, La
Contact:

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

Postby Lakerunner » Mon Mar 29, 2010 7:23 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:
"COONASS COUNTRY"

12'x6' Aluminum /0-540 A/V
"Marines - Providing Enemies of America an Opportunity To Die For their Country Since 1775"
USMC 1964-1970

User avatar
harley
Site Supporter - IV
Site Supporter - IV
Posts: 2073
Joined: Sat Mar 28, 2009 9:53 pm
Location: BREVARD CANAVERAL GROVES COCOA

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

Postby harley » Mon Mar 29, 2010 8:11 pm

I set mine at , 5 - 7'' , below transom.. they seem to work fine at that setting...
14x8 Diamondback / SBC Power.

"ALL THROTTLE NO BOTTLE"

swampdragster
Southern Airboat Member
Posts: 1885
Joined: Sun Apr 26, 2009 10:13 am
Location: Reddick Fl. orange lake.
Contact:

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

Postby swampdragster » Mon Mar 29, 2010 8:45 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..
UNLIMITED SWAMPDRAGSTER DRIVER
QUICK SLICK on every dang thing!!!!

TSnake
Site Supporter - IV
Site Supporter - IV
Posts: 150
Joined: Thu Aug 17, 2006 12:00 am

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

Postby TSnake » Mon Mar 29, 2010 8:58 pm

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.
Micanopy, FL
15' Hoffman Rivermaster
454 Stinger 2.68 3-blade S-series

User avatar
Lakerunner
Site Supporter - IV
Site Supporter - IV
Posts: 1138
Joined: Tue Oct 21, 2008 11:13 am
Location: Maurepas, La
Contact:

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

Postby Lakerunner » Mon Mar 29, 2010 9:04 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.
"COONASS COUNTRY"

12'x6' Aluminum /0-540 A/V
"Marines - Providing Enemies of America an Opportunity To Die For their Country Since 1775"
USMC 1964-1970

User avatar
JAMES
Site Supporter - II
Site Supporter - II
Posts: 8544
Joined: Wed Aug 22, 2007 10:31 pm
Location: cocoa
Contact:

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

Postby JAMES » Mon Mar 29, 2010 9:24 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:
https://m.facebook.com/321ccc/?ref=bookmarks Wp383 268 rotator 3 bld WT/EX 14.4 perdator

User avatar
CJB0470
Site Supporter - III
Site Supporter - III
Posts: 1218
Joined: Sun Jan 04, 2009 9:10 am
Location: Port St Lucie
Contact:

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

Postby CJB0470 » Mon Mar 29, 2010 9:32 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...
"Always do right. This will gratify some people, and astonish the rest"
-Mark Twain

User avatar
nomad41
Site Supporter - VI
Site Supporter - VI
Posts: 4300
Joined: Tue Sep 18, 2007 5:12 pm
Location: Palmetto,Fl
Contact:

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

Postby nomad41 » Mon Mar 29, 2010 9:48 pm

I was told it helps keep inside clean.If you have a little water in boat.It will suck it out.
http://www.flickr.com/photos/nomad41/
Treasurer FAA
Member:FAA, KRVSA ,IRCAA, FSCA,BAHCC,OAA
http://www.suncoastyachtservices.com
14.5x8 Diamondback ,SBC Stroker,2.68 Stinger,Sensenich 82" R
Svcliche@yahoo.com

diamondback0320
Southern Airboat Member
Posts: 1034
Joined: Thu Aug 20, 2009 3:09 pm
Location: palm bay,fl

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

Postby diamondback0320 » Mon Mar 29, 2010 10:07 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
Site Supporter - VIII
Site Supporter - VIII
Posts: 2379
Joined: Tue Feb 28, 2006 11:00 pm
Location: citra fl

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

Postby dantheairboatman » Mon Mar 29, 2010 10:09 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!
Ox Box. Billet gear reductions and clutch systems
Sensenich propellers dealer.
Airboat consulting.
Dan@352 812 2239

User avatar
Lonewolf9029
Site Supporter - V
Site Supporter - V
Posts: 826
Joined: Thu Aug 21, 2008 7:01 pm
Location: Houston,Tx.

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

Postby Lonewolf9029 » Mon Mar 29, 2010 10:22 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
TAA-426 15' x 7' Panther>>>350>>>2.68 Rotator gearbox>>.Sensenich 3bld 80" "R" series

User avatar
Champ18ft
Site Supporter - VIII
Site Supporter - VIII
Posts: 1832
Joined: Mon Oct 03, 2005 6:34 pm
Location: Parrish, Fla

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

Postby Champ18ft » Mon Mar 29, 2010 10:25 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.
13-5' Gilleo Lazer 0-540 A/V 74" Q
ImageImage

dantheairboatman
Site Supporter - VIII
Site Supporter - VIII
Posts: 2379
Joined: Tue Feb 28, 2006 11:00 pm
Location: citra fl

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

Postby dantheairboatman » Mon Mar 29, 2010 10:46 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:
Ox Box. Billet gear reductions and clutch systems
Sensenich propellers dealer.
Airboat consulting.
Dan@352 812 2239

User avatar
Waterthunder
Site Supporter - VIII
Site Supporter - VIII
Posts: 8160
Joined: Fri Jan 21, 2005 3:16 pm
Location: Melbourne
Contact:

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

Postby Waterthunder » Mon Mar 29, 2010 10:47 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!
THE PROOF IS IN THE PROP!

"If you copy someone you will only achieve what has already been done."

http://waterthunder.com/
321-508-5316

User avatar
Champ18ft
Site Supporter - VIII
Site Supporter - VIII
Posts: 1832
Joined: Mon Oct 03, 2005 6:34 pm
Location: Parrish, Fla

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

Postby Champ18ft » Mon Mar 29, 2010 11:12 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.
13-5' Gilleo Lazer 0-540 A/V 74" Q
ImageImage

User avatar
DAGWOOD0520
Site Supporter - VIII
Site Supporter - VIII
Posts: 640
Joined: Thu Feb 15, 2007 3:01 pm
Location: Durant/Plant city
Contact:

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

Postby DAGWOOD0520 » Tue Mar 30, 2010 6:34 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.

User avatar
air-roberts
Site Supporter - IV
Site Supporter - IV
Posts: 1799
Joined: Sat Jun 27, 2009 8:01 am
Location: cocoa / canaveral groves

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

Postby air-roberts » Tue Mar 30, 2010 7:43 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
Air-RobertsImage
experimenting with old step w/t experimental 384.8 c.i. nos 2.68 rotator, 80" three bld ? R
http://www.bcraceengineering.com
Thanks to dave http://waterthunder.com/
New & Used PROPs
WhirlWind prop dealer

User avatar
Southern Style
Site Supporter - IV
Site Supporter - IV
Posts: 5239
Joined: Mon Oct 06, 2008 3:34 pm
Location: Sarno marsh

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

Postby Southern Style » Tue Mar 30, 2010 9:01 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???????????????????????
DONT ARGUE WITH STUPIDITY...THEY WILL JUST DRAG YOU DOWN TO THEIR LEVEL, AND BEAT YOU WITH EXPERIENCE.....
KRVSA Member
SJRAA Member
KCCO

User avatar
watersports7
Site Supporter - III
Site Supporter - III
Posts: 2523
Joined: Mon Dec 01, 2008 10:15 am
Location: Cocoa/PSJ/St Johns River

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

Postby watersports7 » Tue Mar 30, 2010 2:56 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)
12' Predator / Lycoming 4 Cylinder / Old Skool 68x38 Stick Prop
Perfectly said by Lycoming: Some engines purr like kittens. These roar!

User avatar
leo_081103
Southern Airboat Member
Posts: 389
Joined: Sat Aug 12, 2006 4:59 am
Location: Manila, Philippines
Contact:

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

Postby leo_081103 » Tue Mar 30, 2010 4:41 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"
14 ft x 7.5 ft FG 5/16" UHMW-PE
150hp 1.6L (98cu.in.) Toyota 4AGE-20V
78" 3 blade ground adjustable Wood prop (Home built)
3.75:1 Belt Drive (Home built)

swampdragster
Southern Airboat Member
Posts: 1885
Joined: Sun Apr 26, 2009 10:13 am
Location: Reddick Fl. orange lake.
Contact:

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

Postby swampdragster » Tue Mar 30, 2010 9:27 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
UNLIMITED SWAMPDRAGSTER DRIVER
QUICK SLICK on every dang thing!!!!


Return to “Automotive Power Only”

Who is online

Users browsing this forum: Yahoo [Bot] and 7 guests