2010 40hp mercury prop question

1gumbi

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I just got a new used motor and it came with a 10 3/8" by 11p prop. When running wide open it goes 30mph at 5750 rpms and it feels like I have a lot more throttle left. I also had some slippage if I gave it too much gas during take off. If I go with the mercury website recommendation I should have a 10.5" by 14p prop. How will this change things for me? I believe it will give me a more speed and lesss rpm's am I correct?
 

oldman570

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The prop needed is the one the merc website recommends for most boats, but may not be what will work best for your use. By going to a 10.5 X 14 prop, the RPM should drop about 600 RPM or there about. You will get a few more MPH also. The whole setup really depends on the weight of the boat and the total load of it when on
the water. There are several web sites with prop calculators that will give you a better idea as how everything works. I believe that there is one here at the Iboats prop section. JMO
Oldman570
 

Sea Rider

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Do you have that engine well transom height seated ? If giving too much juice to engine during take off, do you have prop slippage or some kind of prop aireation, ventilation ?

If you are running wide open (WOT) engine won't have much more to accelerate unless carb butterflies are not wide open at 180? check.

Happy Boating
 
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steelespike

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We need more info; 2 stroke or 4 stroke? What size/weight/type boat? Is speed by GPS? Only sure way to get speed.
You said "wide open" is 30 mph then said it feels like you have a lot more throttle? Using a 2.00 gear ratio the boat can't go 30 with the numbers you quoted.
We need wot rpm and gps speed. to evaluate your setup.
If you read the sticks at the top of the prop forum it will help you understand prop selection better.
 

1gumbi

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Sorry for not having enough info. I have a 1979 basstracker II its a jonboat with mod-v front 16.5 ft long, Hull weight without motor is 600-650lbs. plus 400 lbs of crew and gear weight. The motor is a 40 hp 4 stroke which weighs 216 lbs with max rpm's of 5750-6000 rpm. It came off a pontoon boat. It currently has a 10 3/8 by 11p prop. the only concern i had was top end speed which was 30 mph per my garmin fish finder with what seemed like a lot more throttle left.
 

1gumbi

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Do you have that engine well transom height seated ? If giving too much juice to engine during take off, do you have prop slippage or some kind of prop aireation, ventilation ?

If you are running wide open (WOT) engine won't have much more to accelerate unless carb butterflies are not wide open at 180? check.

Happy Boating


I set the motor cavitation plate with the bottom of the boat. My transom is 21in tall. I'm using the second hole from the top of the motor mount. I marked my prop before going out with a few scraches on sides and middle. No change. I'm not sure if it was aeration, or ventilation when it slipped. It only happened once so I was careful not to give it too much gas. I should clarify that I only went up to 6000 rpms I was scared to give it full throttle or more throttle.
 

steelespike

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I think a 14" Spitfire 4 blade prop should get your rpm where you want it,should grip better and may conserve your hole shot.
Not having your wot rpm I think the 14 is a good guess. Your rev limiter would likely cut in about 6300 if you wanted to see what you get.
 

Sea Rider

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With current prop running mediun loaded at 6 K and having more throttle to go, rev limiter will kick in very soon. Need to dial new prop for current carrying load and boat type as that prop was dialed to push a pontoon boat. Could check what's the standard factory delivered prop and go from there.

Happy Boating
 

1gumbi

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I just picked up a 4 blade solas from a buddy 10.5/14p he said I could borrow it tomorrow and test it out. I'm putting it on tomorrow and heading to the river. I can't wait. It went pretty good with the other prop. I'm not out for more speed.(30 is fast enough) I just want to make sure I'm not harming my new baby.
 

steelespike

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You probably will net more speed but you can operate at what ever speed/rpm you prefer if it is propped right.
 

Sea Rider

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Test that new prop at WOT with you alone and check max rpm achieved and if rev limiter kicks in or not. Then try same with current previous load and check if inside min, middle max wot rpm factory stated for that engine. Once running between middle or max rpm you can thottle at any speed you like with engine stress free.

Happy Boating
 

1gumbi

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What a difference 3 degrees makes. I ran the 10.5/14p 4 blade prop today. The low range performance and boat control was noticeable when in the faster currents below the dam. As far as top end speed I think I'm to heavy on the pitch. At wot I gained 1 mph but my rpm's didn't go over 5100 rpm's. I took that prop off and will give it back. Its nice to have buddies. I'm leaning towards a 12 or 13 pitch now. Also today when at full throttle I noticed that last Sunday when I thought I had throttle left I didn't. What do you guys think? 4 blade 10.5/12p? Thanks for any replies
 

roscoe

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I don't see anything wrong with the prop you have, except possible some slippage/ventilation on hard accelleration.

if you think the 4 blade is really heaping you that much, then pick one up. But I would not go over 12p.
 

steelespike

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So it appears a 14 is too much and your 11" was right on. Keep in mind when going to a 4 blade prop it is usually suggested to drop one inch in pitch.
Of course you were actually at max throttle so the 4 blade 14" was too much.
I think a 11" Spitfire prop should work.The spitfire is said to not require the pitch drop. I think it will maintain rpm,should improve grip.
The 4 blade is said to stay on plane better at reduced throttle,feel smoother and respond better around the dock.
 

Sea Rider

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If I had slippage, ventilation, aereation would check where is water flow passing at lower leg at speed provided boat runs on flat no wind water cond and engine is well trimmed. Ideal is water flow skimming slightly under small wate deflectot plate, there prop has the best water level for opt prop grip and overall best cornering at tight turns on any water cond.

Happy Boating
 

1gumbi

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Thanks for the input guys. I could really tell a big difference between the two props. The 3 blade made the boat feel quick and the boat felt as if it was floating versus the 4 blade with its better control and responsiveness at low speeds but I could feel the drag when at wot. I wish I could get my hands on a 4b 11p for testing. I got the 14p number from Mercury's site. But I was skeptical. I'm glad I have this site to get some good info. Someone asked about motor height. I was by myself and could'nt get away from the wheel. But I did look for splashing and there was very little. I won't be going out for couple weeks. So I plan on trying to find something local. I'm thinking I'll look for a used 3b/12p or a 4 b/11p I have another buddy with some props but he must have caller ID. Dang it. He hasn't called back. I don't want to buy a prop unless I know for sure its the right one. Thanks guys.
 

1gumbi

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I was looking at my motor today and noticed some numbers just above cavitation plate 13times26 and my gear ratio 2:00/1 what's the 13 by 26 mean?
 

1gumbi

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i decided to go with a 10.8/12p 4 blade solas aluminum. I hope it is the right fit
 

steelespike

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A 13 tooth gear meshed with a 26 tooth.The 13 turns twice to get the 26 to turn once.
Motor turns twice and prop turns once.
 

1gumbi

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Awww! Hence the 2:00/1 gear ratio. I thought it had something to do with max prop size. I was watching a fishing show this morning and the host used his tach to turn down trolling speed on his Mercury. Can I do this with my 2010 40hp merc? Thanks for the info steelspike.
 
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