Boat Performance Question

[croSSed]

I came across a 1979 18' Marlin Tiger Shark with a '79 Merc. I6 140 on it for sale. This is a boat that has always intrigued me, and I was considering buying it. <br /><br />In case you don't know, this boat is a low profile, ski boat (NOT a Tahiti style boat) that you would often see back in the '70's and '80's with a V8 v-drive powering it. Marlin, Advantage, Kona, Avenger, etc. all made versions of this same low profile, 18' ski boat.<br /><br />I would expect a boat like this - if the engine were running right - to be about a 50-55 mph boat at top speed. It turns out that the guy selling it told me the best performance he gets out of this thing is 5000 rpm with a 17 pitch prop, doing about 40 mph! This seems WAY off to me! It sounds about right to me if that 140 were actually a 90 in disguise. What do you all think? Thanks for any help.<br /><br />TG

 

[JB]

Re: Boat Performance Question

Boat topic.

 

[Bondo]

Re: Boat Performance Question

You Might be Right,.......<br /><br />But,....<br />It takes ALOT of Horsepower to get over 50mph,......

 

[Clams Canino]

Re: Boat Performance Question

Weather it's a 140 or a 90 or a 70 - an outboard turning a 17 pitch prop at 5000 rpm, on the same boat, will give the same speed. <br /><br />It's not WAY off - just a little low but not bad. Try a GPS speedometer to get an accurate reading. The big inlines cruise nice at about 4000 to 4200 RMP. That outta be around 25mph with a 17p.<br /><br />-W If you have amore inline 6 questions ask them back on the Mercury board - I don't come here often.

 

[QC]

Re: Boat Performance Question

I understand and agree with Clams Camino's comment about the prop pitch, but isn't that only true if they all have the same gear ratio? Do all Merc OBs of that vintage have the same gear ratio regardless of bhp? Doesn't sound right to me.

 

[Bondo]

Re: Boat Performance Question

If I'm not mistaken,....<br /><br />Back in that day,... The 4" gearcases were All the same,......<br />With the Exception of the 65hp..........which was the Smallest motor with the 4" case.....

 

[QC]

Re: Boat Performance Question

I that's true, then clamboy has it.

 

[croSSed]

Re: Boat Performance Question

I have a lot of examples of why these performance figures don't seem right, but here's the biggest example. Presently I own a 16' runabout which is not exactly a high performance hull. It has a 1987 Merc. 115 on it. I'm swinging a 20" pitch Lazer II. At WOT it turns 5600 rpm, at between 47 and 48 mph, according to my speedo. Doing the calculations at 10% prop slip, that is about right. How does my boat go faster than this low-profile performance boat with basically the same engine? It just doesn't add up.<br /><br />TG

 

[TilliamWe]

Re: Boat Performance Question

Ok, if the math is right, then the engine isn't producing the power, or the boat is way heavy, or has a big hook on the bottom. Have you inspected it up close?

 

[croSSed]

Re: Boat Performance Question

I have not inspected it yet. That's my next step. The guy who owns the boat is sending me the serial # of the Merc. I'm going to check and see if that 140 is really a 90. BTW, my boat, the runabout I describe above, DOES have a hook in it. No matter how much I trim it I can't get the bow out of the water. It was designed that way. If it didn't have the hook in the bottom, it would be really unsafe at high speed. It would get REALLY loose. I can still crowd 50 in it. That Marlin ought to kick its a--.<br /><br />TG

 

[Texasmark]

Re: Boat Performance Question

Hey QC Attaboy. <br /><br />I was going to mention that to Clams, but you beat me to it.....but that's ok. Like my last engine was an inline 115 tower of power. Had a 2:1 gear ratio as I recall. Current one is a 3 cyl 90 and it has a 2.3. Definite difference plus the weight of the engine has to be accounted for.<br /><br />Mark