4 blade on Mercury F20

The Noob Sailor

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First thing first: Please take a look at this link, and take a note especially what the topic-starter JerryS writes:

http://forums.fishusa.com/m/tm.aspx?m=447315&p=1

I'm thinking of changing the propeller on my Mercury 4-stroke 20hp engine. After reading quite a bit on the Internet, I have decided to try out a Solas Amita 4 prop. I have been discussing a bit with fellow countrymen on a norwegian forum, some saying the 4-blade would cause to much drag, due to one extra blade. When reading the thread in the link above, I see that a 4-blade could work just fine on a 20hp motor, as long as the pitch is correct according to the boat weight and hull design. My boat is a Smartliner 170 aluminum boat (check Google pics) and is not a typical V-hull boat. Dry weight on the boat is 460 lbs, and me including gear and gas tank will be aprx 280 lbs. Comparing to JerryS's boat my boat is much easier on the motor, less hull drag I guess. The OEM 3 blade prop on the Mercury F20 is 9 1/2 x 10p. When reading JerryS's thread on the 4-blade topic, I convinced myself I should get an Amita 4, 9 1/2 x 8 prop, since I sometimes will bring wife and kids along on a boat trip, adding aprx 200 lbs weight. But since I mostly go fishing alone, I'm eager to find out if perhaps a 9 1/2 x 9 would be the best fit for the overall setup. Maybe the engine will over-rev and the limiter kicks in with a 8 pitch 4 blade... The latter propsize is also the pitch which is the rule of thumb when converting from 3 to 4 blades. I did check rpm with a tach with the OEM propeller, and it messured 6140 rpms WOT. Speed checked with gps; aprx 24 mph on calm seawater.

Anyone got a hunch which Solas Amita 4 prop would fit the boat and overall setup and use best? 9 1/2 x 8 or 9 1/2 x 9?
 

Sea Rider

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Welcome to Iboats,

What are you trying to accomplish ? OB is already running at max wot range which is 6100 rpm with factory delivered 3 blade oem prop ? Assume 6100 wot revs was while lightly loaded ? If so, add extra family members go for a wot run as previous test and report back achieved max rpm. Will know rpm difference between running lightly and more loaded.

Don't know why boaters likes going for 4 blade props when 3 blades ones perform excellent, just need to dial best pitch for OB to run near or at its max wot range as currently loaded.

Happy Boating
 

The Noob Sailor

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Thanks for quick reply, and thank you for welcoming me.
I actually checked rpms with "family load" and also a slightly bigger load being me and another big guy and his two kids. If I remember correct the Tach did show 5400 rpm on heaviest load. The use of the boat is like 80-90% just me, and 10-20% family load. With family load I guess the Amita 4, 9 1/2 x 8 would be the best choice. But with me alone I'm worried about whether the 8-pitch will cause over-reving or not. The 9-pitch should put about the same load on the motor, as the 10- pitch 3-blade, according to what I read on forums etc. Reason way I want to try out 4-blade is that the engine sees to have a decent amount of tourqe in the midrange, and I believe a 4blade will have more bite in the water and get the boat faster on plane with bigger load. The 3blade seem to struggle with this and typically need more rpm to do the same job. Isn't there a term called propeller slip? I expect the 4blade to have less slip, but then again this probably add more load on the engine, so maybe the 8-pitch is the better choice anyway?
 

Sea Rider

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What's the max HP-OB and number of passengers that your boat can take ? Seee 2 issues : your load varies much between lightly loaded (solo boating) and family/friends oriented (higly loaded) seconfly OB is just a 20 HP. Depending on load, OB runs at both sides of the min 5400 max 6100 wot rpm range

A fixed load is easier to maximize a prop as weight is constant, not variable as in your particular case. Whichever prop you choose will need to be water tested. See if possible to borrow both props from a prop shop without the need of buying them. Don't compare your boat to others, each boat and load is unique, each one its own prop.

Happy Boating
 

The Noob Sailor

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Having the opportunity to check out different props would be nice. Unfortunately I live in a less dense populated area in Norway, and the nearest dealers that can get props from the grossdealer in Norway are 2 hours by car. Second, I don?t think the?ve got 4 blade Solar props in different pitches for Tohatsu/Mercury 20hp lying on the shelves, as this is an uncommon prop to ask for. So I need to try to pin point what would be the best compromise pitch for my setup, since I actually have to buy a prop for watertesting. The gross dealer doesn?t have the 8-pitch anyway, so I don?t think they send a prop through half the country for watertesting, and then get it back and end up with no sale.

If anyone have a hunch for either the 8- or 9-pitch, I?d be grateful to know. The guy on the other forum had the best compromise with the 8 pitch, with a considerably heavier boat setup, thus meaning I probably should get the 9-pitch... But then again, would the 8-pitch be nice to have with heavier load, and maybe would not cause over-reving because of extra blade and more propeller drag? I think 6200 rpm is wot on my motor, I guess that?s when the limiter kicks in...
 

Sea Rider

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Factory max wot is 6100 rpm, rev limiter usually kicks in 150-200 revs over max wot. You need to invest some money if wanting top boating, that's for sure. I see the variable weight issue to be a problem selecting a top prop, for that will need to count with a variable pitch prop which are quite expensive, don't think are available for portable OB's.

Try the 4 blade 9 pitch lightly and heavily loaded and see how it performs.. There are not to many 4 blade pitch props to choose from, are usually for heavier near fixed loads. On 3 blade props there's a wide selection to play with. If with current prop rev limiter is not ckiking in while lightly loaded at wot, why are you worried about.

Happy Boating
 

The Noob Sailor

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Hehe... investment limits was the issue when buying a new OB 3 years ago. My wife was pregnant with our first kid, so the 20hp was an economical compromise. I actually ordered the Suzuki DF30A, which then was brand new on the market. At first the dealer gave thumbs up, but it later showed that he couldn't get hold of the engine I wanted until the next season. After a brief family council we decided to cut $1200 off the OB budget. Last year we bought our first house, so a 30hp OB is quite bit down the list nowadays :)

If you read thorougly, you see it's the 8pitch I worry about. The 9pitch will be spot on at solo boating I think, but maybe not a compromise prop...
 

The Noob Sailor

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I kind of agree with your suggestion, the 9pitch. Maybe the compromise is not in prop selection, maybe mediocre speed with bigger load is... But I have a suspicion that the 4 blade 9pitch will do different things in the midrange rpms.
 

Sea Rider

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Have lots of experirnce maximizing OB props, usually from underpowered OB's. Can say that if you dial a prop to run at it's max wot rpm range, will do excellent at idle, middle to max wot range. But can't go boating efficiently while lightly or heavily loaded with same pitch.

Have seen cases where 2 different pitch props were needed to handle light and heavy weights, not at same time, depends on load. Don't expect a wow speed with a under powered OB while heavily loaded no matter which prop is chosen, there are limits to it.

Can always re pitch half size props at a prop shop if going for a 4 blade 8/9 pitch prop that doesn't work well. Ordering a prop is like ordering a custom suit over the phone, both have to be tested for a perfect fit.

Happy Boating
 
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