Jack plate installation

[Crestliner2008]

I have a rather small rig: 15' Crestliner & 15 HP Yamaha. This rig is underpowered for compliance to local reservoir restrictions. So, I put a Doel Fin on the cavitation plate to get my rear end up on plane quicker and to try to save some gas. It worked just fine! Use to take me 30 - 35 sec. to get up on plane; now less than 5 seconds. Amazing! However....now I have the problem of lost top end speed! I've lost about 8 or 9 mph!   I already have a HD 9" pitch prop, so I figured I'd get a jackplate to try to get the DF up to the surface of the water at top end; the drag of the fin, below the surface of the water, is what I suspect is causing my loss of speed. The jackplate I bought is a "Mini Jacker":

My problem is, how can I temporarily (safely!) afix this jackplate to my boat's transom, so that I can try various engine heights and check my engine's rpm's to stay withing mfg. specs.? (I have the "Tiny Tach" tachometer to tell me when I'm approaching the danger zone.) Any suggestions would be greatly appreciated.  

[RobDar]

...maybe use the third hole from the top and the seventh hole from the top on both sides...that way you can move it up and down two hole locations in either direction????

...looks like that will give you about 3" adjustment up and down????...I would think that 3" would enough to fine tune things. I have never had or installed a jacker but I hear sometimes just an inch makes a big difference.

[w4svc]

I run a 14 foot Jon and a 15 Johnson. I've moved the fuel cell and battery to the center of the boat behind the center seat. This helped.

If you have the fuel cell and battery in the back you may want to try this. I also tried several settings on the trim and found the best setting for 1 or 2 people. If you have tried all these Your way ahead.

[Crestliner2008]

Yes, I already have installed a HD 9" prop; both batteries are in the bow; the portable gas tank in in the mid-ships. Before installing the Doel Fin I was getting 26 mph (using GPS) with 2 fishermen & gear. After the installation I dropped down to 18 mph. I raised the engine 1 1/2" with a wood block and this got me up to 22 mph, so I believe on the right path. I also have installed a "Tiny Tach" tachometer to tell me if I get too close to the max. rpm's. Even with this last adjust raise, the Doel Fin is not up to the start of the keel, at the bottom of the transom, so it's still dragging quite a bit in the water. This is why I've elected to try a jackplate to raise the engine even more and try to re-gain my top end speed. I believe it will work, but we'll see!   Thanks again for the input!

[HPBB]

first off, your getting 26mph out a 15hp? I have a hard time believing that.  :-?I can see maybe 20( and that is pushing it) and that is with one person. If you have enough power to push a boat to 26mph you should have more than enoght to plane it out

I run a 15 foot starcraft with a 15 hp merc with dolefinns and a mini jacker like you showed. I get 15-16 mph by myself and about 13-14 with 2 guys. This is a V hull with floors and decks, 2 TM's, 2 batteries, 15 hp merc (tiller) and 6 gal gas. I borrowed a 25hp merc and only could get 25mph out of it. so I still question 26mph out of a 15hp. but anyway this is what I have found out.

The dolefinn do not put that much drag to slow you down that much.  even if they are still in the water they are made to slice trough the water and not drag it. I will say one thing maybe to much weight is up front. before I put the dolefinns on my 15 footer it would not plane. put the gas up front and the battery. a little help. put the dolefinns on and it planed real quick, but now there was too much wieght up front The bow was digging. ended up having to put the gas and battery in the back again and the speed came up. with the jacker the speed came up about 2 mph. Like i said I top out at 16mph

[Crestliner2008]

HPBB - All I can tell you is what my GPS is telling me!? Alone in my rig I use to get her up to 29 mph, prior to the DF installation. Now, alone I can only get about 24 - 25 mph. Again, using GPS for speed and comparing results with my sonar speedometer - which, surprising to me, is pretty close to each other. The boat I have is the Crestliner Sportsman series, that measures 14'8" in length and weighs in naked at just under 500 lbs.(she's the widest boat of this size made). Running with two batteries, one trolling motor on the transom alongside the main engine. Both batteries are far forward. The 6 gal. gas tank is mid-ships. The load weight is pretty evenly dispersed. Switched from an 11" pitch prop to a 9" pitch when I first got the rig. This change, along with adjusting the trim on the engine, got me from 15 mph (2 guys) up to 26 mph - prior to DF installation. I just want to get back what I had.

[cart7t]

You'd better get yourself another GPS to verify your numbers.

29 mph with a 15hp motor on a boat that's rated, according to Crestliner, for a 35hp?  You gained 11 mph by dropping the pitch 2" and changing engine tilt angle?  Dropping prop pitch will cause you to lose top end speed not gain it even with an engine tilt adjustment.

Get yourself another GPS.  Your numbers essentially mean that boat, properly rigged with a max 35hp motor,would be running in the 45-50 mph range.  not likely.  

[HPBB]

Like I said and like "carp7" stated. Your numbers are way off. I would check another gps. You boat weighs about the same as mine. and same power. I know many many guys that ran 15 hp motors and could never get 20mph, Infact I don't know anyboby that can get  20mph out of a 14 footer with two guys and a 15hp on it. so they moved up to 25hp ( some local lake changed their hp limits) and those guys still are only getting low 20's, infact I am looking to move up to a bigger motor too.

Ok my boat is a  1988 stracraft SS 14'11" V hull alum, 60 inch beam fatory floors, boat weighs about 400 from the fatory, I added front and rear decks. It runs a bow mout tm and a transom TM, two batteries and 6 gal gas, all in the back & a 15 hp Merc Tiller, with DF and a Mini Jacker. the max hp is 35hp.  My boats runns better when the batteries and gas are in the back. Like i said the most I can get is 16mph on GPS. The fastest I know the boat has gone with the 15hp is 19mph and that was with my one friends running it and she only wieghs 115lbs, and only one battery and no bow tm. So I sill have no Idea how you are getting that speed. What kind of GPS are you using?

[Crestliner2008]

Hmmm....interesting. The performance level I was and am getting now was never questioned by my marina at all. And as far as my GPS goes, if it's off, then so is my Nissan and my F-150. I'm using the Garmin iTrek Legend CX - not a cheap unit. But thanks for the input anyway. I'll let you know what my results are come spring, but I'll guarantee you this. I will get that rig up to 25 mph with 2 guys onboard. Whether you believe me or not  .