Prop Question

[locked&loaded]

Good morning-

I've been reading information here for some time, but this is my first post. I'd like to start by saying how informative this site is and to thank those of you with so much wisdom and for your willingness to share it with others.

I just picked up a little 2013 15' polar kraft. It is the Dakota 1578 WT. It has a 15' LOA, beam of 78 and an approximate dry weight of 530 lbs. It is rated for a max of 30 HP, but currently has a 2013 4-stroke 20 HP mercury on it. The engine itself seems to run well, but the boat is a complete turd in the water. When I'm the boat alone, it takes about 10 seconds or more to get up on plane and once it does, it runs OK at 21-22 mph per GPS. Once I put a buddy in the boat, forget about it. It will not get up on plane at all and will just plow water with the bow up in the air at 8-9 mph even if we move all the weight we can forward in the boat.
I'm currently running the stock mercury prop which is a 9.25 10P aluminum prop.

Initially I would think the boat is way under powered, but my previous boat was a foot shorter, with an 8 inch narrower beam and had a 9.9 HP motor and that boat would hop right up on plane and run really well.
I live at 4500 feet elevation and will often fish lakes that are at 7500 feet elevation. I'm assuming most outboards come from the factory with a prop rated for seas level? My questions are:

- will changing to a prop with a different pitch help my situation and if so, how do I determine which prop to use?
- will a hydrofoil help? I've read lots of positive reviews about the hydrofoil on cabelas and amazon

Should I try one before the other or just switch the prop AND get a hydrofil?

Obviously I could trade the 20 HP for a 30 HP, but I'd rather not go that route if there are other options that would help me get up on plane a bit quicker and easier.

Thanks so much for any advice you can offer.

 

[Sea Rider]

Welcome to Iboats,

Some issues ;

-Will need to tach current prop to check wot revs with you alone and with second boater as well to see if inside wot range for that OB.
-At altitude it's a carb jet swap along less prop pitch to compensate some of the bad water attitude.
-Correct transom/engine height is a must have.
-Changing altitude is a killer as opposed to boat at a given fixed altitude, sea level or at 4500 ft.
-2 boating different scenarios between 4500 / 7500 ft.

Happy Boating

 

[steelespike]

Are your results at 4500 ft?
Your Dakota is 530 lbs dry weight You and a buddy the motor,gear gas and battery your probably over 1000 lbs.
Guessing your rpm(not a great idea) using prop calculator indicates about 5600 rpm You really need a tach to confirm rpm and
to tell if the prop change gets the desired results. Your wot rpm rating is 6100.
Assuming your at 4500 I think a 8" prop should get you planed with room to spare to help at 7500.
If your changing altitude a lot I don't think a jet change will work.
We do need to be sure your well setup.Have you tried different trim settings, Any excess weight placed or eliminated to help with planing.

 

[locked&loaded]

Thank you for the replies.

This week I have run the boat at both 4500 and 7500 feet elevations. When I'm in the boat alone, it will get on plane after 10 seconds or so at 4500. I cannot get it to plane at all, even if by myself at 7500 feet.

A friend had a 4 blade 9.25 7pitch prop that I put on and took for a short ride today. Although the motor reved-up quite a bit more as compared to the 10 pitch prop, the boat still ran terribly and would not get on plane at all.

I'm going to pick up a 3-blade 8 pitch prop on Monday and give that a try.

The motor height appears to be perfect. I have a 21 inch transom and the cavitation plate sits just slight below the bottom of the boat.

Could this be an issue other than just the wrong prop? If so, other things i should be investigating? This is frustrating as you can imagine. The boat is only 18 months old and the gentleman i bought it from appears to have taken very good care of it, broke the motor in properly, routine maintenance, etc. He figures the motor has approximately 25-30 hours on it.

Do you feel that a 20 hp motor on a boat of this size and weight is just not enough and weight vs HP is disproportionate?

Thanks again for the help!

 

[steelespike]

At sea level with a 10" prop if it can make 6000 rpm. mph would be about 23-25 mph
Your problem is the elevation; at 4500 your down about 13%.and thats if the jets were right for 4500. Your motor
is too rich at 4500 and at 7500 your down about 24% plus way too rich jets.
The 7" prop under ideal conditions makes only about 16 mph at 6000 rpm
​ You can't jet for 7500 because it will be too lean at 4500, bad for the motor.
​ I'm no elevation expert but it seems to me that if you jet for 4500 and try a 8 or 9" pitch. maybe make about 18-20 mph
at 4500 and maybe plane at 7500. You would have to keep in mind below 4500 you would be too lean.
Keep in mind that the initial 4500 ft rpm (5600) is the result of a calculation and not an actual tach reading.
It is possible a 30 hp may be needed. to make a reasonable 7500 speed.

 

[Sea Rider]

Install an induction tach to play with prop pitches, you're definitely over revving that OB while playing with less pitch, need to know by how much ? Check with Merc Marine to see if they have elevation jets kits, will work much better along prop pitches. It's not same boating at sea level with underpowered engines that doing same al altitudes. The only issue I see is that if jetting and proping "about" right for 4500 ft, as this is not exact rocket science, don'e expect same performance at 7500 ft, it's a 3000 ft completely different scenario.

Happy Boating

 

[locked&loaded]

I really appreciate all the good information, fellas.
I'm wondering at this point if I'll still be frustrated after trying a 8 or 9 pitch prop. I'm trying to get my hands on a tiny tach so I'm not making guesses at rpm.

After all my frustration with it this weekend, I've sure been kicking around the idea of a new 30HP etec ..... but I'd sure rather use the motor I already have, but only if I can get it to run as I hoped it would.

If the engine itself is running as it should, can there really be that big of a difference in how the boat planes and runs with a 8 or 9 pitch prop as compared to a 10? If you think there is, then I'll give them both a try. If you think the difference will only be nominal at best, then maybe my best route is a 30hp?

What do y'all think?

 

[locked&loaded]

I also forgot to mention that I have tried all the different trim settings with both the 10 and 7 pitch prop and although some seemed to be a bit better than others, none of them were close to satisfactory.

 

[steelespike]

If you go to the etec I think it will adjust to the elevation changes.
Disregarding the hp difference the etec will have more guts when trying to get on plane.

 

[locked&loaded]

Do you all expect that I should expect a big difference if I were to try a 8 or 9 pitch prop as compared to the 10?

 

[Sea Rider]

Playing with prop pitches at sea level with underpowered engines as in your particular case is a different story than doing same at 4500-7500 ft of elevation. With less pitch at sea level will achieve better hole shot than with 10 pitch, but will surely over rev, will move more load better too. At elevation the equation changes completely, thin/less air, engine strugles.

Before venturing on a new 30 HP. Get a Tiny Tach, check for elevation jet kit for 4500 ft, once installed play with 8-7 prop pitches. Jet kits starts at 3K ft elevation and increments sizes at each 3K ft steps. OB still won't perform same as at sea level, but much better than if doing nothing. Set manual trim for AVP plate to ride parallel to water level wnen combo is on plane.

Check with other boaters using E tecs, EFI OB's if being electronically controlled makes a whole boating difference. When going to 7.5-9K feet of elevation at the Andes my MPI car that's very fast at sea level turns into a complete pig, height issues...

Happy Boating

 

[locked&loaded]

Thank you.

I just ran a new 25 HP evinrude etec on my boat and I am pretty astounded at the difference between the 2 motors. It is literally a night and day difference and the etec shoots that boat almost immediately on plane and runs very, very strong.

I'm going to run a 30 HP etec tomorrow to see if there is much difference between the 2 and if it's worth an additional $800 over the 25HP.

Thanks for all the replies, fellas!

 

[SkiDad]

I wouldn't spend the extra if your happy with the 25 performance

 

[steelespike]

There are 25 and 30 hp performance reports at the etec sight.
Considering the elevations your dealing with the 30 may be the way to go.
Your down about 24% at 7500 ft. That puts you at about 20 hp with the 25.

 

[Sea Rider]

Would be interesing to install an induction tach if boat doesn't hacv a center console along intsruments to check wot runs while lightly loaded at 4500-7500 ft of elevation with standard deliverd prop. You could max prop if in need, assume you'll not be boating solo all the time and would like to take boaters for rides as well. For that scenario will need a different prop pitch. A tach will tell if propped right for different boating situations at elevations.

Personally would go straight for the 30 HP model.

Happy Boating

 

[locked&loaded]

I'm going to pick up an 8 and 9 pitch prop today and hopefully run them later this afternoon or tomorrow morning on my 20HP merc. If they don't perform satisfactorily, then I'm going to go with the 30hp etec. Have a local shop willing to give me $2000 on trade for the merc (which is about 600 less than I know I could sell if for myself ..... IF it were spring or summer months) and will make me a decent deal on the new etec, which is $4695 with electric start and electric tilt and trim.
I'm hopeful an 8 or 9 pitch prop will work well on the merc so I don't have to shell out a bunch of extra cash, but I'm willing to do what I need to, to make the boat enjoyable.

Thanks for all the good help, fellas.

 

[locked&loaded]

Thanks for all the help. Now I know the 2-stroke vs 4-stroke issue has been probably been beaten to death, but based on my recent experience, the newer 4-strokes pale in comparison to the newer 2-strokes.

I found a prop shop that allowed me to take several different props and I ran a mercury 20 HP 4-stroke, a yamaha 25 HP 4-stroke, and an Evinrude 25 HP 2-stroke on my boat with several different props for each motor and the Evinrude blew the other motors out of the water in nearly every performance category. I found a shop that took my 20 HP mercury in trade toward a new Evinrude and I could not be happier.

 

[Sea Rider]

Congrats, unfortunately high altitudes compromises severely standard 2 strokes and 4 strokes OB's performances. Etec, Efi, TLDI engines rules boating at high altitude elevations as opposed to carbed OB's.

Happy Boating

 

[steelespike]

There are 25 and 30 hp performance reports at the etec sight.
Considering the elevations your dealing with the 30 may be the way to go.
Your down about 24% at 7500 ft. That puts you at about 20 hp with the 25.