Speed with 9.8 Nissan

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Get a Suzuki 9.9. Change the ECU and inlet plate a you have a true 20 hp motor. You may also want to look at the prop you’re using
 
I’m in a similar situation. 1/2” plywood decking, two 27 series batteries(placed in front of middle bench), 3 gallon fuel tank, tackle and other crap, Lowe 1467UV, 2013 Tohatsu 9.8hp, 4 stroke carb. 17mph gps.
Engine was 3-4” under keel, I raised it, actually put it a little above keel, speed dropped. 13-16MPH, depending on trim setting.
Moved batteries to the stern, struggling on hole shot. Gonna move one battery forward.
If that doesn’t help, different propeller.
My manual has pictures about cavitation plate placement.
 
Remove any non essential weight. Your boat would work well as designed and produced but when you added a lot of structure to the boat you have substantially increased the hull loading on the available planing surface. You have modified a very narrow boat!! The less planing surface you have the more drastic each weight change and placement will be. You cannot compare your numbers to another 14' boat that is wider with more planing surface.
If the structures you put in the boat are fairly easily removable do that and then run the boat light and see what you get. If you are serious about getting performance out of this combination you will have to do this. There is no magic pill answer.
You only have 10hp to work with and that gets taxed pretty hard when you load them down with weight.
When testing always get a two way average in a current situation, one way numbers are useless.
Raising your engine on the transom will not give you much in this situation. Trim angle, overall weight, and weight placement will. Again.....lose the weight and check. Change trim angle with each change you make.
If you are set in stone about running with this much weight you may need to drop some pitch in the prop with the load you are trying to push. An RPM check at WOT will tell you.
 
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Doyou have a tachometer to tell you the rpm? It could simply be that you don’t have the right prop pitch for the load……..knowing what rpm you are getting would tell you that……
 

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