Alumacraft Tunnel hull prop recomendations Yamaha f70

SweatNredsSweatNreds Posts: 51 Deckhand
I have been sent though the ringer the last few days and my head hurts now. I have e-mailed powertech and alot of other folks about their props and suggestions. 3 request including from power tech and all 3 had different answers. I am not opposed to any brand as long as my goal is met in the long term.

Issue: Top end speed sucks and motor is over revving
Currently 24-25mph
Stock 13 5/8 x 13 pitch prop
WOT 6600-6700 rpm
2010 yamaha f70 4 stroke
2012 Alumacraft 18ft cc tunnel hull boat with sponsons
aprox weight loaded 1800 lbs
Micro Jacker Hydraulic jack plate (not installed yet) 4 in set back with 5in lift possible

Goal: Go faster :spin. I see the yamaha service bulletins with this motor on similar boats/weights and it should be moving along alot better than it is. I have not installed the JackPlate. Never had one either. I know this will be a problem when I do install this when it comes to the stock aluminum prop that is on the boat and probably any prop with out cupping.

Practical goal is 30 plus mph at around 6100 to 6300 WOT rpm speed. However with this in mind running in a channel or deep body of water with perfect conditions.

Any suggestions on the correct prop to start with?

Thanks for any advise.


  • Pat LPat L Posts: 440 Officer
    When you say stock, I assume you mean aluminum as these motors do not come with props, its whatever the dealer puts on it. If it is then yes you will get a lot of slip from it on a tunnel hull. You could probably stay with the same pitch but go with a stainless prop with a decent mount of cup in it. As far exactly which prop its still kind of a crap shoot as these motors are still pretty new to everyone and every set up is different.
  • SweatNredsSweatNreds Posts: 51 Deckhand
    Yes Stock Aluminum dealer installed Prop.

    I hear ya on the crap shoot.
  • SweatNredsSweatNreds Posts: 51 Deckhand
    Well PowerTech proposed a SWW 4 Blade pitch of 12. Went to my local dealer today and he had a pitch of 13 on the shelf. He also told me my rpm numbers are wrong and that my gauge is off. Said that the rev limiter would kick in at 6150. I don't think this is the correct but I will have to get this looked at. He did tell me that he would exchange it for free until I got the right one so I pulled the trigger. I will be installing the jack plate today and then off to the lake to see what it will do.
  • Pat LPat L Posts: 440 Officer
    Your dealer is wrong. Most 4 stroke and 2 stroke Yamahas are limited at 6150 except for the F70 and its operating range is between 5300 and 6300rpms. I cant remember where the limiter kicks in at but its probably around 6500, it may be 6700 but I'm not sure I would have to check. If your dealer doesnt believe you tell him to look in his product information guide. It is still possible your tach might be off and that is something he should have checked before delivering the boat to you along with having the correct pitch prop on the boat.
  • SweatNredsSweatNreds Posts: 51 Deckhand

    Well back to the drawing board. This prop is not the answer what so ever. No take off no mid range rpm 4500 now. This thing is just to aggressive. 24 mph with this one.

    old prop yamaha aluminum 13 5/8x13 ran 29mph. My numbers were off on my above info.

    Any suggestions?
  • ken2ken2 Posts: 10 Greenhorn
    Just called a buddy of mine that has that motor to see if he knew where the rev limiter kicked in.
    He said he thought it was right around 6400. So as Pat mentioned, your tach may be off a bit.

    I'm not a fan of the SWW for the small 4 stroke motors. The prop has a HUGE amount of cup, and its just overkill.
    Thats why a 12 or 13 pitch prop will hold you back so much on RPM.

    These motors need a prop that spins up easily during holeshot.

    I like the Turbo Quest. (i dont sell turbo but I'm getting them soon) and the Powertech SCD3.
    Of the two, I'd kinda lean toward the Turbo.
    15 pitch is where I would start with either of them.
    It has good bite, so it should still work decent with the jackplate, and it turns up easy during holeshot.

    load is obviously going to effect this motor a lot.
    The one my buddy has on a 17ft Scout runs 6200 RPM 41.5 MPH with him in it, And if I get in (200 lbs) he gets 5800 and 38.5 MPH.
    These are back to back runs, so literally the only difference is 200 lbs.
    Some boats will be more sensitive to weight then others.
    Ken with PropGods
  • SweatNredsSweatNreds Posts: 51 Deckhand
    Thanks Ken I appreciate the info. I been racking my brain all afternoon and was thinking along the same lines of staying with a 3 blade so it turns up quick and take a chance on a medium range prop with the jack plate and hope it did well enough.

    My tach is off because the folks that rigged my boat has the setting off behind the tach it self. Had I known what I know now those gauges wouldn't even be on the boat but I guess you live and you learn.
  • SweatNredsSweatNreds Posts: 51 Deckhand
    Dealer still pushing 4 blades. I don't think he has any and is trying to avoid ordering.

    Power tech pfs4x14 absolutely worse than the other. Barely 3600 rpm another dog

    A lot better scd4x13 6400 to 6300 rpm 29mph can run with jack plate all he way up with no issues. No hole shot not very good midrange power. But a hole lot better. Other new issue with this one. Lots and I mean a lot of bow rise. I need a blade with some stern lift vs bow lift.

    Thanks for the advice and if you have any suggestions please let me know.
  • 72soa72soa Posts: 314 Officer
    Setting up tunnel hulls is a challenge. I think you will need a 4 blade prop. In addition, the tunnel hull design you have is a slower hull. More friction. Keep that in mind when looking at top end speed.
  • ken2ken2 Posts: 10 Greenhorn
    Some people put the jackplate all the way down for holeshot.
    That is not necessarily the best spot.
    It may get up much easier with the motor lifted a little. It will have a sweet spot for holeshot, just as it does for top speed.

    We tried every prop I could get my hands on with the F70 on a 17 scout, and still ended up putting a hydrofoil on there to help with holeshot.
    I'm normally not a huge fan of those things, but we had the engine height set up so that the foil is out of the water at a fast cruise anyways so its not hurting anything.
    It made a big difference on that little scout.
    Ken with PropGods
  • VertigoVertigo Yankeetown, FLPosts: 617 Officer
    Your tunnel hull is pretty inefficient for top speed. I'm speaking from experience, having owned and now owning two tunnel hulls and having gone through the same drill with props. I'm guessing that with the F70, no matter what prop you use, you'll be doing very well to get much better than 29mph. I'd quit worrying about getting more top end and be happy with a good hole shot and decent acceleration.
  • SweatNredsSweatNreds Posts: 51 Deckhand
    After about 5 more props I can concur on the top end deal. The best prop to date has been the aluminum k series 13 pitch prop but it over revs. I went back to just messing with 13 pitch props in 3 blade configuration only. So my delimia now is I want to run a SS prop. The only 4 blade prop that came close to what I was looking for was a power tech scd4 in 13 pitch. It ran 29/30 mph as well. It got 5800 rpm max and was a dog on take off as well as acceleration. If I got the scd3 which in return will have less blade area would this increase my rpms and in crease acceleration/hole shot? Next move will be getting a 14 pitch aluminum k series from Yamaha then look into selling the motor and upgrading to a 90 2 stroke.

    Got to have a chance to keep up or at least run with the rest of the boats on tournament day.
  • duckboneduckbone Posts: 285 Officer
    I don't know how this turned out for you but I have done alot of stuff with aluminum tunnel hulls. An aluminum prop is not the way to go a 4 blade stainless lifter prop is what you need. Also you should search venting your tunnel on some duck hunting sites then call Baumann props in Texas.
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