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Prop question

Discussion in 'Boats and Motors' started by yakfish, Sep 13, 2005.

  1. I bought a boat a few weeks ago and I think I need to get a new prop for it. It is a 1977 15 foot fiberglass bass boat with a 75 horse evinrude. the prop that is on now looks really old and pitted there is a chunk broken off the tip of one of the blades. also it take forever to get up on plane when there is more than 2 people in the boat, but once it does it flies. I took it out for a rip myself and the speedomitor on the fishfinder said I was doing 55. basically I am trying to figure out how to choose a prop, and i will keep this older one as a spare. is there a way to measure the pitch off the one that is on there . and I don't know how the different pitch effects the performance. and is bigger diamiter always better? There are alot of questions here and I need some educating! thanks
    Good fishing
     
  2. I am not nearly an expert on props so I will not get into making any suggestions. However it sounds like from your description of the performance that the prop you have is more designed for speed than power, which would contribute to slow planing. However I don't think I have ever heard of a 75 HP on a fiberglass boat making 55 MPH.:eek: If that number is anywhere close to being right then I would say that you have a pretty adequate setup.
     

  3. Orlando

    Orlando Set The Hook!

    You can't properly prop a boat by the speedo, they are not accurate. Speedos are notorious for giving high readings, unless it is a GPS. Besides you need to know what the rpms are at wide open throttle to prop it properly. You don't want to be spinning that motor at higher rpms than it is supossed to be. A chunk out of the prop can make a huge differnce in performance. You may just need a new prop in the same size. What pitch and size is it? There are numbers inside the hub. Also does the motor have trim and tilt? If so you need to tuck the motor under , push throttle all the way forward then start triming up the motor. This will give you a quicker holeshot.
     
  4. I wish it had a power trim but you can only lower and raise the motor by hand. The fishfinder with the speed reading doesn't always work either so I wouldn't be a bit suprised if the speed reading was inaccurate. this is more of a flat planing hull which doesn't push nearly as mucvh water as a v-hull boat once on plane so I am not too suprised at the speed (if it is right). I know of a guy who had a boat similar to this one who said that it would do 65. but who knows unless there is an accurate way to measure the speed. Right now though I am more conserned with the sloooooow take off we went out with the four of my family in the boat and it took close to 2 minutes to actually get up to plane. Like I said earlier it goes fine with just 2 but any more than that it just boggs on the take off. the only thing I can think of is the prop.
    Thanks
    Good fishing
     
  5. You select a prop for all around performance by loading the boat in what you consider the normal load. Usually two people and their equipment. You then observe the RPM's to see where they are with respect to factory spec. As far as getting on plane faster, you may want to install a fin on the cavatation plate. I had to change props on my rig and now it is within 150 RPM of spec.
    Keep in mind that an aluminum prop will flex and cost you somewhere between 15 to 20%. That is why you see SS props on the larger HP motors.
     
  6. Orlando

    Orlando Set The Hook!

    Don't want to be the aerer of bad news but it could be mechanical also. I once had a older outbord that didn't want to plane but would run great once up. Come to find out one cylinder had no compression. May want to do a compression check, those old Johnson, Evinrudes are known for dropping cylinders. Hope it's judt the prop. You might try going to a marina, they sometimes have used props you could try. Good luck
     
  7. I never thought about it being commpression which could be possible. I'm not sure thought beacuse it does just fine getting up on plane with just two people. Ishould take it to a boat place and have someone look at it though
     
  8. If you have acess to a decent library system there's a book on boat props that will tell you more than you'd ever need to know . Can't rember the name of it off hand, some thing like the Complete Book of Props, might also find it's name on the wooden boat site under book sale page. Any ding in the prop should be fixed, runnig with an unbalanced prop is hard on the seals at the least, and the problems can compound fast depending on the extent of the damage. Diameter and pitch are stamped somewhere on the prop, usually inside the hub, though years of wear, exhaust and corosion may make it hard to read. The 2 and 3 cyl looped charged OMC engines of that era were Very good motors, way advanced compared to the v-4's of the decade before. While 55 does sound unlikely, it should have plenty of power for that size boat. Today there are more prop makers than ever but the old standard is still Michigan Wheel , very likely they made the prop that came on your motor new. Try contacting them or finding info they publish as a guide to where to start. Pitch, material and whether it's cupped or not will be your main determinate as diameter is limited by the manufacture. If memory serves me right pitch ranged between 15 and 19 inches for those motors used in runabout applications. A 15" would give you great power, excellent if used for water ski or heavy loads, though speed would be limited to about 30 max. Conversly a 19 would be your speed prop with a loss of 'hole shot' or heavy load capability. Again, my memory isn't that great, a trip to the local library may also yield an old manual that should have a prop guide table as well.

    Good Luck, Chris
     
  9. UFM82

    UFM82 The one others want to be

    2,053
    281
    1,268
    We've been here before and it wasn't pretty. :S

    Anyway, lets clear up a few things. Without a tach and without knowing your WOT spec, you take a chance of either destroying your engine or destroying your engine. That is, you can either over-rev, which can cause immediate failure, or you can lug the engine, which is a long term killer. You need to invest in a tach before you start messing around. You can get a good quality tach like a Teleflex for around $50 and it will be money well spent.

    Props work best when they are 100% and many props will not be 100% even after a few outings. Props with visible damage are far from 100% obviously and can really be a huge performance killer. It's pretty amazing to see just how badly a damaged prop can drain performance away.

    You can remove the prop and look at the hub back where the nut goes- many props will have the part# or even the diameter/pitch stamoed in it. Or, a quick trip to the prop shop can give you the same info. If you plan to continue to run that prop, take it in and have it repaired. It will make a big difference in how the boat runs.

    That being said, it was mentioned above that speedos on boats are unreliable. That is very true- most read significantly higher they should. Why? They run off water pressure and that can vary greatly from boat to boat due to pitot placement, bottom contours and even temp of the water. When you hear someone say that their boat runs 60mph and they are going off their speedo, it's pretty safe to assume they may be getting to 50 or less.

    Your engine is a 75 hp and that was rated at the flywheel back then. So, at the prop your getting less than that- probably about 8% less. Plus, the bottom of your boat is probably pretty flat. You say you have no Tilt&Trim? That doesn't help either.

    For a good holeshot and then good top speed T&T is almost a necessity. I would look into a bolt-on version like a CMC unit. But that's down the road.

    Take a look at this site: http://www.rbbi.com/folders/prop/propcalc.htm

    It can help you determine how fast you can travel in a boat with a few pieces of info. I'm gonna guess that your boat will probably run in the high 30's with everything running right, maybe even low 40's. I can pretty much guarantee that 55 is not possible in your boat and that your speedo is off.

    You are in the area where a stainless prop will help you. Cabela's and Michigan offer small SS wheels for your engine and they can help your performance. Keep that damaged aluminum prop as a spare or have it repaired- you can have a small prop like that fixed for about $50.

    Get the tach and get the numbers; then we can work on getting the performance out of her. I'm thinking 40mph would be a good number on that ride.


    UFM82
     
  10. Unless that boat is shaped like a canoe.....75 isnt pushing it 55. I would agree..
     
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