Increase horsepower on Zodiac

[TOHATSU GURU]

Re: Increase horsepower on Zodiac

Take off power is dramatically different between the 30 hp 2 cylinder and the 40 hp 3 cylinder and the top end difference is awe inspiring...But, that is between these two particular models. In general, there usually isn't a lot of difference as fbpooler has pointed out. Engine weight is not an issue to anyone except Zodiac due to paranoid liability concerns...And that is because the majority of the people who manufacturer, sell, service and repair inflatables don't use them and have zero practical knowledge about them. You should hear the crap I'm told by manufacturer reps and no neck inflatable dealers Most of these people have no experience at all in boating, run around for a day in a brand x inflatable and then become the source of all knowledge.

I wouldn't exceed what a manufacturer plate says's you can put on a boat. The reason I wouldn't do that is because I don't want to get a ticket on the water or get sued when some Mook on a PWC runs into me and screams "look he was overpowered so it's his fault too."

 

[Sea Rider]

Re: Increase horsepower on Zodiac

It is also a guess to compare a RIB to a SIB performance wise.

I'm just stating that I''m fully pleased with the top performance of my 18 and Joe is completely dissatisfied with his 30 HP performance being my combo a lot heavier when he should be having lots of fun with that nice combo.

In you have a well air topped sib to factory specs once on water the performance will be near same in flat water conditions, it's all about tube rigidity. Before switching to ribs have used for long time 360-380 alu floor sibs and went intantly into plane with my oldie Tohatsu 18, those last 2 came transom height factory modified. The 18 performs so well that will be passing it to my new 450 rib and expect to have a slight least performnace than with 420. Once you install TT will definitely know what's happening with your sib/engine combo, post wot results.

Happy Boating

 

[fbpooler]

Re: Increase horsepower on Zodiac

Sea Rider, I also find it interesting that 30 HP on a 12 1/2 ft boat does not please the OP. I suspect a prop change and proper weight distributation should allow the boat to plane rapidly. I can see where installing a heavier engine could worsten acceleration if weight distribution is the problem which would make him a candidate for trim tabs.

We run a 10 ft Avon and have used both 9.8 and 15 HP motors. There is only a few MPH difference in them, but the 15 is enough to make the boat squirrely to control and hard to keep on plane without having to watch for oversteer. The hull is designed for moderate speed to obtain the best handling.

Our 14 ft fiberglass has been run with 15, 18, and 25 HP engines, and there is only about 3 MPH difference in top speed. The weight of the 25 was detrimental to performance due to the stern heavy condition caused by the weight of the engine. Sometimes the installation of a larger engine lessens the enjoyment of the rig. If you chopped the throttle too quick, water would pour in over the transom with the 25.

Our 17 ft fiberglass with 30 HP is only about 3 MPH slower than the same boat with 40 HP. You pick up 8 MPH by going from a 30 HP to 60 HP engine according to some boat tests done by others. Acceleration is moderate with the 30 but plenty fast enough to be satisfying.

A 17 ft vee bottom we owned would hit about 40 MPH, but that was with an 85 HP engine.

This experience always puts a smile on my face when someone wants to know how much better the performance will be with a 30 rather than 25 HP engine. Once you have enough horsepower for a clean plane, you really have to put on the horses for a large improvement.

Satisfactory acceleration is a relative thing. You can prop a boat to literally jump out of the water with proper weight distributaion, but then over rev the engine at speed. Those calling for using a tach to check the performance are right on target.

Any 12 1/2 ft boat turns into a squirrel to control when you add enough horsepower.

The OP also wondered about the drag of a boat top. With a relatively light 12 1/2 ft boat, operating at speed with the top up is asking for trouble. It could flip the boat in a split second.

 

[Sea Rider]

Re: Increase horsepower on Zodiac

I find strange too that a 380 sib with speed tubes that only weights 85 Kg doesn't perform well with a 30 HP. We use 380 alum floor sibs as surf shuttles that weights 78 Kg with Tohatsu's 30 HP and that combo flies with 4 surfers, surfboards + driver. I'm not a big fan about using trim tabs, doel fins, once you have sib top inflated, set transom engine height correctly with well trimed engine, it's just a matter of ballancing load towards bow.

Trim tabs helps less powerful engines to plane faster boats which should plane well without them if placing max recom rated HP on transom. A must have too if lousy skipper is not directing the crew where to properly sit to ballance boat correctly. With time, different loads under trial & error will learn to ballance efficiently your combo.

Happy Boating