TNT w/40 hp merc and WRONG prop

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Denon Osterman
Posts: 515
Joined: Thu Jun 22, 2006 8:58 am
Location: toronto, CAN

TNT w/40 hp merc and WRONG prop

Post by Denon Osterman » Thu Jul 12, 2007 8:45 pm

so i finally got the controls fixed and was able to drive it in forward, and going along at around 4000 rpm (redline is a very low 4500, but i wanted to keep it safe, because i had a prop that was in SEVER need of a re pitch(this 4000 rpm was achieved at around 3/4 throttle) i got mid/high 30's speedo reading...flooring it for a second saw low 40. with the under pitched prop it didn't even really "get up" on plane...one second you were sitting there putting along, the next you were on plane cruising along, rapidly gaining speed :D

When i get a working motor(a 35 johnson w/ptt is in the works), and the right prop...I'll borrow my freinds GPS and give a thorough update, hopefully that will be at the end of the summer...I'm hoping for high 40's.

a side note on the waves...around 20-30 it smacks around like you wou;dn;t beleive on mediocre size waves...then somehow glides over massive ones like they weren;t even there...but get going above 30 mph and everything smoothes out. turns on a dime at low speeds, havn't tried turning that sharply at high speeds, but it track very well...on rails, as they say...though my fin is a BIT bigger then the plans call for, because of the choppy conditions.

Overall i'm extremly happy...can't wait for a working motor and PTT :D

Denon

Craig_Bay
Posts: 20
Joined: Wed Aug 22, 2007 7:52 am

Post by Craig_Bay » Wed Aug 22, 2007 12:02 pm

Hey, just wondering what pitch you are using, and what pitch you plan on getting. I have a merc 40 on a tnt as well, so id be curious to know what sort of performance you get with the right prop. I was thinking an 18" would be about right, but havnt even gotten my tnt in the water yet, so dont have much of a clue. Thats a suprisingly low redline for that motor. I have a '81 40HP and its redline is around 5500 I think. Anyways, good luck with getting the right pitch.
-Craig

Denon Osterman
Posts: 515
Joined: Thu Jun 22, 2006 8:58 am
Location: toronto, CAN

Post by Denon Osterman » Wed Aug 22, 2007 12:46 pm

I have a '81 40HP and its redline is around 5500 I think.
im not sure if yours is before or after they made the shift, but right around 1980 my "40" turned into the post 1980 "35", and a new "40" was made from the 50's block(much bigger)

anyways, that engine had some issues, so i switched it out for a new johnson 35 hp w/ ptt. right now, it has a 10x17 3 blade on it, and im getting it repitched(today, actually), to a 10x21, because the redline is 5500, and its turning around 6100( :shock: )at WOT, i get about 40 mph out of it now(speedo, not GPS)

ill let you know how it goes when i get it re pitched

Craig_Bay
Posts: 20
Joined: Wed Aug 22, 2007 7:52 am

Post by Craig_Bay » Wed Aug 22, 2007 1:22 pm

They actually made a 2 cylinder Merc 40 up to 1983, which would have been somewhat similar to the 400. Mines just called a Model 40. They didn't make another 40hp until 89 I don't think, which was when they turned it into the big 4 cylinder with power trim and tilt and such that bumped the weight up to 190 or so. The 400s from the very early 70s are notorious as being one of the worst mercs ever made. Not surprising that you had trouble with it. Thats interesting to hear that you're still spinning that huge RPM even with a 35HP. I guess i better go for the 19" for sure, which from what i gather is the largest standard pitch they make for midsized mercs. Good luck with the repitch.
-Craig

RoyH
Posts: 133
Joined: Tue Aug 30, 2005 6:54 am
Location: Norway

Post by RoyH » Thu Aug 23, 2007 5:58 am

If it was my boat, I would mount a nosecone and cleaver prop on that new motor. Thn you can lift the engine so that the prop-center is 1" under the bottom. The speed would realy jump up.
But it will be useless for waterskiing of course.. 8)
I'm just a starteled bunny in the headlights of life..

Craig_Bay
Posts: 20
Joined: Wed Aug 22, 2007 7:52 am

Post by Craig_Bay » Thu Aug 23, 2007 6:27 am

RoyH wrote:If it was my boat, I would mount a nosecone and cleaver prop on that new motor. Thn you can lift the engine so that the prop-center is 1" under the bottom. The speed would realy jump up.
But it will be useless for waterskiing of course.. 8)
I was under the impression that cleavers are only for seriously high speeds. I cant see this boat ever getting higher then high 40's. Took it out for the first time last night. Ran ok, but i find the throttle is sort of an all or nothing device. Mayby something wrong with how i have it hooked up. Also, had it trimmed wrong. Wish i had power trim in this boat, but i guess that would add a healthy amount of mass. Going to try moving the gas tank to the front tonight. The corners of the back end are just slightly under water w/ me and motor as is.
-Craig

Craig_Bay
Posts: 20
Joined: Wed Aug 22, 2007 7:52 am

Post by Craig_Bay » Thu Aug 23, 2007 4:39 pm

Went out with the GPS tonight to do some runs. Ive got a 40HP with a 15" prop. I would think im underpitched, and the noise that the motor makes when given wot (only for 5 seconds) would seem to confirm this, BUT the GPS is telling me im only hitting 32, which is a little disappointing. Though it feels plenty quick. :P My prop is a little dinged up, so i could be catching quite a bit of slip. Anyway, im going to put in a call to TINY TACH tomorrow to try and get a tach rushed to me, then am going to go buy a couple props that ive found used around here for cheap. A 17" and a 20". Should probably go order a carb kit tomorrow too. wheee. Ill keep you guys updated.
-Craig

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Caber-Feidh
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Location: Battle Point, Leech Lake... tundrasota

Post by Caber-Feidh » Fri Aug 24, 2007 6:07 pm

Cleavers are most useful if the hull develops aerodynamic lift. They generate little bow lift on their own.

Scott
Posts: 308
Joined: Sun Sep 17, 2006 3:21 pm
Location: Canada, Southern Ontario

Post by Scott » Tue May 20, 2008 12:49 pm

Craig_Bay wrote:Went out with the GPS tonight to do some runs. Ive got a 40HP with a 15" prop. I would think im underpitched, and the noise that the motor makes when given wot (only for 5 seconds) would seem to confirm this, BUT the GPS is telling me im only hitting 32, which is a little disappointing. Though it feels plenty quick. :P My prop is a little dinged up, so i could be catching quite a bit of slip. Anyway, im going to put in a call to TINY TACH tomorrow to try and get a tach rushed to me, then am going to go buy a couple props that ive found used around here for cheap. A 17" and a 20". Should probably go order a carb kit tomorrow too. wheee. Ill keep you guys updated.
-Craig
Whats up with the larger prop? Go smaller, bigger diameter props create prop drag. Get a little 10.25"D x 14"P prop or something around that size to create less drag thus a little faster. I could see you going way way faster in your TNT w/ a 40. PM Andrew N for tips on what you should do. His TNT does 37.5mph w/ a 25hp.

And also, whats your engine's mid section length? 15" or 20"?
Thun.der.bolt
ˈTHəndərˌbōlt

A flash of lightning with a simultaneous crash of thunder.

An ignition system of early Mercury outboards that not only is a nightmare to diagnose but also improves a marine mechanics competency.

upspirate

Post by upspirate » Tue May 20, 2008 9:19 pm

Scott,I believe he was talking about a larger pitch,not diameter prop to get more speed & reduce RPM

Scott
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Joined: Sun Sep 17, 2006 3:21 pm
Location: Canada, Southern Ontario

Post by Scott » Fri May 23, 2008 5:33 pm

Oh whoops...
Thun.der.bolt
ˈTHəndərˌbōlt

A flash of lightning with a simultaneous crash of thunder.

An ignition system of early Mercury outboards that not only is a nightmare to diagnose but also improves a marine mechanics competency.

Craig_Bay
Posts: 20
Joined: Wed Aug 22, 2007 7:52 am

Onwards and Upwards

Post by Craig_Bay » Thu Jun 05, 2008 6:29 pm

Sooo, my tnt now has a '64 Merc 500 on it. Before you become too horified, i should mention that it only weighs 141 pounds. It is an incredible motor. I had it out for the first time today. At the moment it has a thin blade 10.5x17p brass "record" prop on it, and that managed to push it to just over 43 on gps trimmed to the nuts. I feel like there's more to come if i can get the trim and prop set up just right. Im going to try to jack the motor up an inch as ive heard that those pre 70s motors actually had 16.5 inch shafts, though the transom plate looks to match up with the bottom of the boat reasonably well. We'll see. Id really like to manage 50mph in this boat someday. :) Take it easy.
-Craig

Scott
Posts: 308
Joined: Sun Sep 17, 2006 3:21 pm
Location: Canada, Southern Ontario

Post by Scott » Thu Jun 05, 2008 10:20 pm

Nice top end! Glad to hear that motor weight is low too! You must get to 50mph. Raising the motor would be a good idea especially with that prop of yours. It wouldn't really throw off the center gravity ethier thus being so light.

If you're a daring person you could contact Ron Hill (google his name) and get him to make a custom prop for you. Like a 3 blade cleaver, ET or Chopper.

By the way, what did you do with your 40hp?
Thun.der.bolt
ˈTHəndərˌbōlt

A flash of lightning with a simultaneous crash of thunder.

An ignition system of early Mercury outboards that not only is a nightmare to diagnose but also improves a marine mechanics competency.

Craig_Bay
Posts: 20
Joined: Wed Aug 22, 2007 7:52 am

Post by Craig_Bay » Wed Jul 02, 2008 9:42 pm

Hey again. Sorry, i dont check this nearly enough. I ended up selling the 402 to a local guy for parts. It wasnt a great motor. I still havnt got the boat set up quite as well as id like. Im up to 45mph though. I think I may have my prop cupped to avoid cavitation while getting up on plane. I wish i had power trim and such but I suppose that would add a lot of weight that the tnt wouldnt be able to take. One day this boat will run 50. haha.
-Craig

Scott
Posts: 308
Joined: Sun Sep 17, 2006 3:21 pm
Location: Canada, Southern Ontario

Post by Scott » Wed Jul 02, 2008 10:59 pm

Why not measure the shaft length to make sure its that 17.5 inches. If true you've got an excuse to jack your engine up that remaining 2.5 inches or more!
Thun.der.bolt
ˈTHəndərˌbōlt

A flash of lightning with a simultaneous crash of thunder.

An ignition system of early Mercury outboards that not only is a nightmare to diagnose but also improves a marine mechanics competency.

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