Prop suggestions for 35 hp on 1648 flatbottom

[james__12345]

I have a 1648 lifeline flat bottom with a 1979 johnson 35hp. With 2 people and probably another persons weight worth of gear and console I was only going 21 mph, and I honestly expected more. The other reason I think this may be the wrong prop is I hit 20 mph at around 3/4 throttle and any more throttle after that only seems to rev the engine and only gains 1 mph. The current prop is a 10 1/2 x11. And came on the motor when it was on an old trihull fiberglass boat. I have a 10x13 that came on the non running who that was on the flat bottom when I got it. Should the 10x13 be closer to the right prop for this boat? Also what speed do you think this boat should be able to hit with a 35? I know I need to check rpm to be sure about the prop but my tinytac got messed up and I haven't had a chance to get another one yet

 

[Sea Rider]

Wot max speed is relative, must know exact wot revs as loaded. Probably need a less pitch prop to pull wot revs up but without a tach OB is already dead...

Happy Boating

 

[james__12345]

It's a loweline boat, just noticed that autocorrect mistake

 

[Texasmark]

35 hp on a 1648 ought to fly.......take all the pitch you can buy! Install and run the 13 and while you are at it, look for more.

 

[james__12345]

The 10x13 only got me up to 22 mph. I have a feeling there is something more going on here. Checked compression today and it's good (118 to 120 on both cylinders) I'm going to try to find a tac tonight.

 

[jimmbo]

The 79 35hp has a WOT range of 5000 - 6000 rpm. OMC prop charts from 1981 show a 13" props was for 13' - 15' boats with a speed range of 26 - 34mph. The 11" was listed for 14' - 17' boats with speed range of 19 - 27 mph. You really need to know what RPMs the engine is attaining

In the early 80s my dad had a 1980 35hp on a 14 ft mirrocraft car topper, with a 10 X 13 it did about 31 - 32 mph

 

[james__12345]

No one around here carries a universal tach so I ordered another tinytach (my old one had a damaged wire and is lost right now anyway).

I replaced the coils and plugs (noticed the old coils were loose where the laminated metal goes through the other part). Took the boat out again after that, and while I was out I tried all the other trim pin locations, and I already had it in the best one. Didnt gain any speed and didn't notice much difference in how it ran from the new coils so the other could must have been ok despite being in rough condition. I was amazed though at how big a difference having the wrong trim made while testing the other spots.

I just noticed that I hadn't mentioned in this thread that the motor is a long shaft but the boat had a short transom. By the end of the day tomorrow I will have a raised transom in the boat to see how much that extra drag was costing me. I didn't think it would be much, but it may turn out to have been more than I expected.

 

[Sea Rider]

If lower leg was achieving huge water splashes out transom which didn't noticed and now have disappeared, will notice the big difference with a medium range OB. Raise transom accordingly for water flow to skim right under upper water defector plate, that's the best water bed height for prop to achieve top forward thrust.

Sit OB at 90?, distribute deck weight evenly, go for a wot run. Once OB runs at the sweet transom height, go for a prop maximization to get all those 35 horses out of that OB.

Happy Boating

 

[james__12345]

I raised the transom last night, and I went a little too far. Tonight with motor trimmed right, the prop was blowing out at around 17 mph (lower if there was ANY chop on the water). I trimmed it in one hole, and i was able to get to 25 mph before the prop lost its bite in dead calm water about 22 in a light chop. I'm excited now to see what it will do once it quits blowing out.

I also gained around .75 mph on my kicker which is also a long shaft and was brought up by the new transom. I'm thinking I can probably bring it up more, since the motor won't plane on that motor I think i could run it with the plate right at the surface at its max speed and still be ok, but I'll need to dig in to that a little.

Unfortunately it will probably be Friday before I get to try it with the transom cut back down. I guess I'll start at an inch and go from there. Maybe the tach will be here by then.

By the way, thanks a lot for the numbers jimmbo. I always figured there had to be something with that type of info on it, just never had found it.

 

[james__12345]

10x13 only got 4700 rpm. 10.5x11 is back on and I'm about to run it. We'll see what happens.

 

[Texasmark]

The 79 35hp has a WOT range of 5000 - 6000 rpm. OMC prop charts from 1981 show a 13" props was for 13' - 15' boats with a speed range of 26 - 34mph. The 11" was listed for 14' - 17' boats with speed range of 19 - 27 mph. You really need to know what RPMs the engine is attaining

In the early 80s my dad had a 1980 35hp on a 14 ft mirrocraft car topper, with a 10 X 13 it did about 31 - 32 mph

Funny, I had a 14' Mirrocraft in the '80's time frame I bought new also with a used 30 Merc, late '50's vintage electric start I picked up from a nice old widow. Had to do nothing to the engine but mount it and the cables and controls and she would fly. Had no instruments so don't have any numbers.

 

[james__12345]

The 11 got me to 5200 at 25.5 mph. Looks like that's about as good as it's gonna get for now. May see what other props are out there as options for later. The 6hp kicker is only getting 4200. Definatly have to start looking for a prop for that one.

 

[Texasmark]

I raised the transom last night, and I went a little too far. Tonight with motor trimmed right, the prop was blowing out at around 17 mph (lower if there was ANY chop on the water). I trimmed it in one hole, and i was able to get to 25 mph before the prop lost its bite in dead calm water about 22 in a light chop. I'm excited now to see what it will do once it quits blowing out.

I also gained around .75 mph on my kicker which is also a long shaft and was brought up by the new transom. I'm thinking I can probably bring it up more, since the motor won't plane on that motor I think i could run it with the plate right at the surface at its max speed and still be ok, but I'll need to dig in to that a little.

Unfortunately it will probably be Friday before I get to try it with the transom cut back down. I guess I'll start at an inch and go from there. Maybe the tach will be here by then.

By the way, thanks a lot for the numbers jimmbo. I always figured there had to be something with that type of info on it, just never had found it.

Playing with performance without PTT is a bummer. You need different trim angles at different times in your transition from idle to WOT maxed out. For you, gotta tune for what you do most and take your licks with the rest....if you are after most speed, you have to suffer through long hole shots. If you want to pop your neck hole shots you have to suffer the loss of top end....or get a ported prop.

On blowing out, cupped props are made to reduce/prevent that.

 

[james__12345]

Just realized I hadn't updated this after trimming the transom. I took one inch off and everything is working fine now. I also messed with the trim a little and am getting nearly 27 mph out of it as of now. I'm about to be adding a bit of stuff to the boat soon, a floor, front deck, a live well and bait well and a new console so that will likely change a little but I'm happy with how it's doing. I did forget how much rougher a ride a flat bottom is though and have realized that I need to cross wakes at much less of an angle than with my old modified v.