Mercury 18"...is this a long shaft or short?

I have an old Mercury 9.8 hp motor and when I mesure the shaft length from the inside of the U clamp to the cavitation plate it's 18". The 16' aluminum deep V boat I am looking at has a 20" transom heigth. Will this work? I will eventually purchase a larger motor but I would like to use this one if I can. Thanks...Matt

Re: Mercury 18

Re: Mercury 18

it should work fine.

Re: Mercury 18

Re: Mercury 18

It may not be deep enough to plane. 2" above the bottom of the hull will probably cause ventilation when you start picking up speed. As long as the water pickup is underwater, you shouldn't have any problems running at displacement speed.

The 18" shaft is their short shaft.

Re: Mercury 18

Re: Mercury 18

I have to agree with mj.

First of all, a 20" transom is 5" higher than a 15" 8). The manufacturer deliberately set up the engine for a 15 so you are 5" above the design goal.

A 9.8 is a fishing engine and trying to run that engine with the top 2 inches of a 9 inch diameter prop out of the water isn't going to get it. You're not even going to get on plane.

The boat wil start out ok for the first few mph and then all of a sudden the prop will start sucking air and the rpm's will go out of sight and the boat speed will drop to near nothing and once it starts sucking air it probably will continue (regardless of boat speed...now a couple of mph) until you kill the throttle.

Now if you had a high hp engine with a high performance prop and all you could blow your way to plane and high speed.

You need to glean something out the fact that they make it that long for a 15" transom. They did that for a reason.

Don't do it.

Mark

Re: Mercury 18

Re: Mercury 18

It's an OMC, but the principles are the same. Go to www.old-omc.de/index2.html.

There are 3 pictures on that page showing the location of the antiventilation plate with respect to the bottom of the boats. All are significantly below the bottom.

Just think about it. On plane, there is no water behind the transom. If no water there is air. Air in your prop is ventilation and the boat will stop and the engine will overrev. :/

Mark

Re: Mercury 18

Re: Mercury 18

should work fine since a 9.8 has no chance of plaining a 16' boat. I run a 6hp short shaft kicker on my 16' boat and It works great for trolling and will push my boat about 6-7mph but I also have a 300# V4 engine hanging on my transom also. The main issue I have with the short shaft kicker is reversing, since the prop is only 75% below the bottom of the boat it shoots water up the transom and it doesn't back up very well because of that.

Re: Mercury 18

Re: Mercury 18

Well rancher, no insult intended, but 6 or 7 mph means that you are in the displacement category and water is still touching the transom. If the guy wants to do that, a 5hp will suffice.

Mark