12.5' with 9.8 running slow!

[Sea Rider]

Re: 12.5' with 9.8 running slow!

An engine loses 15% or bit more power at each 1 K meter elevation, so more than 30% off at 7 K. Elevation jet kits comes in different elevations, so pick the one that matches your current one. At very high altitudes will need to play with less prop pitch for engine to deliver higher lost rpm.

A re jetted engine for high altutide use must be re jetted again with factory jets once at sea level. If boating constantly on high elevation lakes, 2 carbs, sea level & high lake use is a must have, just swap carbs.

Happy Boating

 

[fbpooler]

Re: 12.5' with 9.8 running slow!

With four people in a boat that size with 20 HP, I would be surprised if it would plane at sea level, much less at a higher elevation. You are expecting too much out of the rig.

You are going to destroy that engine loading it like that. You are also going to have transom problems in time.

 

[saabsaviorlee]

Re: 12.5' with 9.8 running slow!

With four people in a boat that size with 20 HP, I would be surprised if it would plane at sea level, much less at a higher elevation. You are expecting too much out of the rig.

You are going to destroy that engine loading it like that. You are also going to have transom problems in time.

Wow I didn't know that it was that overloaded. I guess it makes sense given the weight and elevation that the results would be what they were. I know capacity plates can be misleading but if it says 1600 pounds weight and 20hp what should be a realistic expectations of load?

 

[fbpooler]

Re: 12.5' with 9.8 running slow!

Use it in a life saving situation and it will support 1600 lbs, but you are not going to push it very fast with any engine. Our 14 ft x 5 ft beam fiberglass vee bottom is rated for 650 lbs including people, motor and all gear for instance. The 17 ft x 6'-8" beam foam filled hull boat is rated for 850 lbs person weight with a maximum total weight of boat, motor, acc, and people of 1400 lbs. No way you could plane it with that much total weight without a heck of a large motor on it, say 40- 60 HP. Both have much firmer bottoms than an inflatable.

The 20 HP rating is what the transom will handle when the boat is on plane. They probably have a fudge factor in that to protect themselves against lawsuits.

I would guess that 400 lbs or so would be the max if you actually want to plane the boat. No idea of how much horsepower it would take to plane it with that load. Elevation restricts the horsepower developed by the engine, but HP to speed are fixed and depend on weight whatever the elevation.

I forgot the formula for hull speed, but your 6 MPH must be about it. It would be faster with a longer boat. You can push a boat over hull speed, but the power requirement doubles with just a small increase in speed.

The transom is lighter loaded when the boat is allowed to move through the water than if it were tied to a fixed object for instance. All the power must be handled by the force applied to the transom in the fixed situation while some of it is bled off if the boat is moving.

We can help you get the most bang possible with your rig, but we cannot avoid physical limitations.

The next time you have it in the water, try it with one aboard, then two, etc. and observe the speed obtained. You can arrive at a balance for the rig under different loadings in a practical vein. You will find that once the rig will not plane, the speed will drop drastically.

 

[Sea Rider]

Re: 12.5' with 9.8 running slow!

That's why I always go boating with generally 2, not necessarily with max engine rated, but with top air inflated tubes, engine well trimmed, correctly height seated on transom and perfectly ballanced sib/rib, all 4 points will make a nice boating difference...

Saabsaviorlee, if you are running an oldie with non thru hub exhaust, your boating will be very limited, those props are difficult to find so will have tech issues if demanding more engine rews playing with prop pitches.

Happy Boating

 

[fbpooler]

Re: 12.5' with 9.8 running slow!

Sea Rider, that is a perfect setup unless you have a larger size rig. We run an 11 ft and a 10 ft inflatable with 9.8 or 15 HP engines. The 11 ft planes much easier than the 10 ft due to longer waterline length. I suspect if the OP gets his 12 1/2 footer balanced out right and does what you suggest, he will have a very nice rig.

With just one aboard, the handling of our boats is not nearly as predictable as they are with two aboard. The 9.8 HP is about perfect for our purposes with either boat.

 

[Sea Rider]

Re: 12.5' with 9.8 running slow!

Sea Rider, that is a perfect setup unless you have a larger size rig

Been runing a Evinrude 9.9 on a OMC 305 & Quicksilver 340 sibs rated for 15 HP. A Tohatsu 18 on a 360-380 sibs rated for 20 HP, 420 sibs/ribs rated for 30 HP and soon on a 450 rib rated for a 40 with same 18, with just 2 and well ballanced inflatable no need for a larger engine.

All these mentioned inflatables have had their transoms properly height modify in the way of removable or fixed shims, trasnom chop downs, both engines working at sweet height leg spot. Both engines have achieved + 200 more rpm just by having tail riding with minimun leg drag, this translates into faster planing along the 4 tech pack described before and of course on near flat seas.

If you boat solo the handling is unpredictable because you must sit on any of the side tubes unballancing sib, if you sit in middle deck toward front the ride is much confortable and stable. 2 mates sitting on tubes will ballance better than 1 doing same, just a proper weight distribution issue.

Happy Boating

 

[fbpooler]

Re: 12.5' with 9.8 running slow!

Agreed. A tiller extension would solve the problem, but with the short hulls, things go unstable pretty fast with either motor at speed. Slight tiller movement can introduce poor performance as there is not much hull in the water and once they jump on plane, speed increases rapidly. Of course, slowing down would solve the problem

 

[saabsaviorlee]

Re: 12.5' with 9.8 running slow!

Sea Rider, fbpooler, thank you both for your advice! I'm learning so much about boating that I never would of thought of. How would you suggest I balance the load? Any thoughts on bowbags or something to move weight forward? I have the cooler under the front seat and try to move the battery as far forward as I can when using the trolling motor with the outboard. I normally sit rear and right with buddy front and left. We're planning a fishing hunting trip soon so I should have an update on how the merc 20 performs with the doelfin and the lower elevation bass lakes. Any guess if I'll get on plane with three 200lb fellas? Wish my blazer hadn't crapped out on me or I'd take her out this weekend. I'll be sure to upload pics if I happen to tag a deer when we go out (over the counter archery only deer hunt). I'm just picturing how funny it would look pulling up to the boat ramp or beach with a mule deer hanging off the bow. Thanks again!

 

[fbpooler]

Re: 12.5' with 9.8 running slow!

I doubt seriously that you can plane with three people and all the gear you have in the boat. It appears from the picture that the forward seat is some distance from the bow, so arranging weight forward may be a problem without proper planning.

Several of the posters have discussed placement of weight and other conditions with you and they are right on. The fin may be a problem as it is just more drag in the water when running at displacement speed.

The only way to find out how the rig performs is to put it in the water and try it out. It appears that you now have the engine problem solved, so checking your prop pitch and arranging weight distribution are next in the order of determining just what the capability of the rig is. It would surprise me if it would plane with three people, but the only way to find out is to try it.

Early on you were concerned about time to plane. Once the weight is properly arranged, then you will see what the time to plane is. Do not expect it to jump on plane rapidly with a loaded boat.

I don't see where we can help you further. Determining what the rig is capable of will have to be a hands on project. Now you are at the stage where you will need to familiarize yourself with what the limitations are.

 

[Sea Rider]

Re: 12.5' with 9.8 running slow!

Saabsaviorlee, you're welcome, that's what the forums are for. Don't know if you are a feather or heavy weight champ when boating solo. When boating solo myself used to have a fuel tank & bow bag secured at bow on my previous 380 alum sib to add some weight at bow.

Key was sitting on side tube a bit forward towards bow with extended arm to a position that could turn tiller well. Knew this was not the best position for me as I'm a 1.80 mt & 100 kilo boater, this awkward driving position unballanced sib to a side. Finally ended resting my back on side tube and sitting on top of a custom air floor matching full interior deck. Perfect fit for driver comfort as well as most passengers that dislikes sitting on tubes for extended boating time periods. Ideal for children sitting.

Your boating position with 2 is correct. But none of both examples works ok if sib is under inflated, engine badly trimmed, incorrect propped or engine with issues not delivering high rpm at wot to plane fast. If with any of this issues or a bunch of them, sorry, near mission imposible to plane 3-200 lb mates....Some ballance examples.

Happy Boating