boat is porposing

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FishingFreak13
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boat is porposing

Post by FishingFreak13 »

I took my boat out for the first time on my vacation and was not happy to see that I was porposing like crazy. it wasn't because I had it trimmed to high it was doing it when it was trimmed all the way down. The motor is set at the 2nd to last highest holes. Anything over 35 mph and it is like riding an enraged dolphin. I can't lower the motor anymore as there is no room for the last holes. I didn't have the issue on my old motor. the back isn't over loaded only things in the back are the 3 batteries and the fuel tanks and an anchor. The front has a bow mounted maxum trolling motor some gear but like i said i never had the issue with my old motor. I have a 2001 135 dfi. I'm thinking that I might have the wrong prob on the motor which might be causing it. It is only a 17 or 19 inch prop but can't remember what the diameter is maybe 13.5 aluminum prop. If i go to a SS prop with a 21 pitch would that help with the porposing. I can't move anything else to the front and like I said my old motor a 1991 mercury 135 was fine. Any suggestions. Not going to put trim tabs on right away.

Brian
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scarkner
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Re: boat is porposing

Post by scarkner »

Was the old motor a two stroke and the new motor a four stroke?

It's the shaft length the same? Prop itself may be sitting lower in the water on the new one.

I can't imagine that changing the prop itself would help (unless it slows the boat down).

Really crazy question, is the transom good? Any possibility it it moving under load?
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lape0019
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Re: boat is porposing

Post by lape0019 »

A prop could make a difference but there has to be something else going on here. Did your old motor have a 4 blade prop or are the 17 and 19P props what you had before?
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67ZL1
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Re: boat is porposing

Post by 67ZL1 »

I would say it's a engine height issue first then prop issue second. Is your cavitation plate above the water when at speed? If not raise your engine till it is. Second, what are your RPM's at WOT? You should be 55-5600 with your new engine with the AV plate above the water. I think a 17p 4 blade or a 19p 3 blade will be a good starting point for props, both in SS.
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FishingFreak13
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Re: boat is porposing

Post by FishingFreak13 »

I checked the cavitation plate this morning with the help of a family member and they said it wasn't I checked it at my cruising speed which was right around 36 MPH according to my Humminbird GPS. That was without trimming it up at all because the moment i trim it up it porpoises like crazy like if i had it really trimmed up at WOT. My RPM at WOT was right around 5300 which was bang on and doing about 46 MPH but it isn't safe as it porposes even more without trimming out. I was checking on the mercury prop selector and it was telling me that for just the best general performance would be in SS they reccomended a temptest plus, trophy plus, vengance, High Five, Spit fire and laser II all in 19 pitch. Dont know which one would be better. I'm taking it to my marina tomorrow to have it lifted and than I'll go from there.


Brian
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67ZL1
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Re: boat is porposing

Post by 67ZL1 »

I would raise the engine so that the plate is roughly 1" above the bottom of the hull. This should be a good starting point. You are likely going to see an increase in engine rpm so if those numbers you provided were with the 19p then you may have to step it up to a 20 or 21. Out of the props listed I would say the Tempest would give you the best speed but may suffer on the hole shot.
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FishingFreak13
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Re: boat is porposing

Post by FishingFreak13 »

When you say raise it 1 inch above the hull do you mean 1 inch when its fully tilted down? I'm assuming so.

Brian
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67ZL1
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Re: boat is porposing

Post by 67ZL1 »

FishingFreak13 wrote:When you say raise it 1 inch above the hull do you mean 1 inch when its fully tilted down? I'm assuming so.

Brian
Everyone does it a little different, but the proper way is to tape a ruler or straight edge to the bottom of your hull. Use you trailer tongue jack to level the straight edge. Next level your engine cavitation plate. Measure the straight edge distance to ground and cavitation plate to the ground. Use those numbers to determine how much to raise your engine.
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67ZL1
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Re: boat is porposing

Post by 67ZL1 »

Yes sorry, cavitation plate 1" above bottom of hull.
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FishingFreak13
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Re: boat is porposing

Post by FishingFreak13 »

After having it taken to the shop I was told that they couldn't raise it anymore as it would start cavitating like crazy. So we decided to put a 21 pitch merc high five ss prop It helped with the porpoising at WOT not trimmed but I still couldn't trim it at all. We than decided we would try dropping it down a notch and seeing what happens. Just took it on the water and it was spraying water out both sides of the motor like crazy. If we brought it back up one notch and than put a hydro foil on it would that help at all. I'm starting to get annoyed because it definitely isn't weighted to much at the stern since its not even maxed out for HP. If anyone has any thoughts or ideas it would be great.

Brian
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CBB
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Re: boat is porposing

Post by CBB »

Could it be a weight issue and you need to put some of your weight forward in the boat. Very common on small runabouts and we would put the gas tank in the front or our buddy.
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67ZL1
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Re: boat is porposing

Post by 67ZL1 »

Did they raise the engine up at all? Kinda hard to cavitate with the plate below the waterline. My cavitation plate is 2.5" above the bottom of the hull on my rig. Every rig is different I know.
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FishingFreak13
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Re: boat is porposing

Post by FishingFreak13 »

CBB it shouldn't be a weight issue its an 18 foot bass boat that can take up to a 150HP motor so we are underpowering it but had an older 135 on it before and it ran perfectly. 67 no they didn't and I didn't really push them on it because I'm hoping I can trust a certified merc dealer to know how the engine works maybe I should push them To raise it just to see what happens because at this point we really don't have anything to lose. I'm on vacation at a cottage and haven't been able to fish for 3 days going stir crazy up here because I'm to scared I'm going to damage the motor which is the last thing I want to do.

Brian
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67ZL1
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Re: boat is porposing

Post by 67ZL1 »

This is how your engine should look running down the lake.
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FishingFreak13
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Re: boat is porposing

Post by FishingFreak13 »

I'll double check it in the morning by having my dad follow behind me in his boat. He said it looked like that but wasn't 100% sure cause he wasn't right behind me. He said it looked like that but you just had tons of water shooting out the sides of the boat. Right now I have a 3 blade 21 pitch prop on but the dealer is going to order me a 4 blade 14x19 prop to try and see if that helps more.
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