guess how fast it will go now?!?!

[toyvette81]

m just curious....
ok so i have a 89 sea nymph 161 have been running the johnson 40vro for 3 years but today i got a 98' mariner 75hp, nearly brand new less than 50 hours on it. i see some people getting 50mph out of their 60hp's on these and similar boats. what are some of your ideas of how fast mine will go?! after this swap. on a good smooth day with just me i can get about 30mph now with the 40hp and one of my cylinders is low at 90 psi the other is at 135. im hoping for 55mph!! maybe more? all three cylinders on this new mariner tested at 140psi
can not wait to put it on tomorrow! ill post a pic of the boat with this to give you an idea of the size and shape of hull. there is a 17 pitch aluminum prop on it now.. the 75 mariner came on a 73 model tri hull (heavy) and the motor ran 5200 rpm's on the lake today so i imagine i will need a 19 or 21 pitch prop... i don't want to over rev the motor since my boat is sooo much lighter than that old tri hull...ill probably go with stainless too. im trying to get every mph possible out of this little thing just for fun...
thanks

 

[Bart Sr.]

Re: guess how fast it will go now?!?!

Make sure your transom is rated for the higher horsepower.Speed is great when it is safe.

 

[jay_merrill]

Re: guess how fast it will go now?!?!

Ditto on checking the capacity plate. I would be surprised if that boat is rated for 75hp. I also wouldn't want to be in it at 55 mph.

 

[5150abf]

Re: guess how fast it will go now?!?!

I believe they actually are rated for 75, mine is and it is an 81.

A stated check your transom, you are calling on it to take 100% of rated capacity so be absolutely sure it is 100% and unless you have replaced it I would bet that it needs done, 20 years is pushing it.

We aren't trying to pee on your parade, only trying to keep your new motor off the bottom of the lake.

Actually stand on the cavitation plate on the motor, if the transom moves anything more than an 1/8'' it needs replaced, I can bounce on my motor and the boat moves on the trailer but the transom doesn't move at all.

At full throttle there is hundreds and hundreds of pounds of force on that so when they go it almost always at WOT and it is almost alway catastrophic, imagine your motor running full throttle only attached to a small piece of the transom, it would be very ugly.

 

[ezmobee]

Re: guess how fast it will go now?!?!

That boat sure doesn't LOOK like it could be rated for a 75 so I'll take 5150abf's word for it. Going to be a rocket ship with that on it.

 

[5150abf]

Re: guess how fast it will go now?!?!

Had my boat for 8 years ,the plate is faded beyond reading now but I am almost certain it was rated for 70-75 horse.

Mine is an '81 Sea-Nymph Fishin machine with an '81 Evinrude 50, casting decks,2 livewells ect so it is pretty heavy and runs 29 GPS.

I was kinda suprised to see yours ran 30 with a 40 horse, is that an empty jon by chance?

Oh and I would say you might get another 5 mph with the bigger motor, let us know what it does.

 

[_chris_]

Re: guess how fast it will go now?!?!

Well He said he was testing it today, I think he either went out and it had so much power he flipped it and is searching the bottom of the lake for it or he is having a blast hauling butt around the lake.

 

[toyvette81]

Re: guess how fast it will go now?!?!

lol no i didnt flip it and sink it haha... i did take it out though... i only got 45mph ( i need more wheel!!).. but thats with a slightly bent and chipped aluminum 17pitch prop... the motor was screaming with that and not accelerating any more im sure with a 21 it will pick up a few.. mabye to 52 mph.
the boat is rated for a 60hp, but the 75 is only 30 more lbs (theres always room for 30 lbs haha)
we have done this to boats for years, it will be fine. my transom is good and sturdy.
what pitch do you guys recommend.. i didn't have the tach hooked up today to see just how much it was screaming though tomorrow i will take it out i have the tac hooked up now i read that this motor dosn't need to go much over 5400 rpm
thanks!

 

[CheapboatKev]

Re: guess how fast it will go now?!?!

Dont think it's the 30 lbs thats the issue..It's the additional 15 horses that create "X" amount of stress on the transom..

Now if you dont go all out all the time..maybe...But just once too often and I hope your wearing brown pants when it goes..lol

Really be safe

 

[109jb]

Re: guess how fast it will go now?!?!

I had a 16 foot Smokercraft about like that boat and it had a 60 hp on it. I could get about 35-38 mph out of it.

 

[jay_merrill]

Re: guess how fast it will go now?!?!

Dont think it's the 30 lbs thats the issue..It's the additional 15 horses that create "X" amount of stress on the transom.

He isn't moving up 15hp - it would be a 35hp jump, which is nearly twice what he has on the boat now.

 

[toyvette81]

Re: guess how fast it will go now?!?!

hes talking about the additional horses from the rated horsepower at 60
and im looking now props are HIGH!!

 

[toyvette81]

Re: guess how fast it will go now?!?!

hes talking about the additional 15 horses from the rated horsepower at 60
and im looking now props are HIGH!!

 

[BWR1953]

Re: guess how fast it will go now?!?!

So you're overpowering by 25%... not recommended.

 

[toyvette81]

Re: guess how fast it will go now?!?!

oh it'l be ok, there all under rated just like most everything on earth that has a weight/power limit like a rope, carabiner, chain, strap, wench, towing capacity on everything, bed weight limit on a truck, etc.... because you know people will push the limit on everything, so nothing can be set exactly at what it will hold or stuff would break all the time. so im willing to actually push it a little bit and i feel i will be alright with 15 more horses than whats called for.
and i don't drive my boat like i stole it either, even with the 40hp i ease on it, i never just slam the gas down, so i certainly won't do that now now, it would sink it haha....
but we got it on and ready in a hour...
and shes Fat in the rear now! haha

 

[26aftcab454]

Re: guess how fast it will go now?!?!

oh it'l be ok, there all under rated just like most everything on earth that has a weight/power limit like a rope, carabiner, chain, strap, wench, towing capacity on everything, bed weight limit on a truck, etc.... because you know people will push the limit on everything, so nothing can be set exactly at what it will hold or stuff would break all the time. so im willing to actually push it a little bit and i feel i will be alright with 15 more horses than whats called for.
and i don't drive my boat like i stole it either, even with the 40hp i ease on it, i never just slam the gas down, so i certainly won't do that now now, it would sink it haha....
but we got it on and ready in a hour...
and shes Fat in the rear now! haha

I am disabled from exceeding the "limits"

 

[fixb52s]

Re: guess how fast it will go now?!?!

Just remember the HP rating is based on a transom that was NEW. What condition is yours? Transoms can/do snap if overpowered.

Now if you were going to rebuild it to take some more punishment, than I say have at it.

Just be careful (my God, I sound like my mother )​

 

[WAVENBYE2]

Re: guess how fast it will go now?!?!

Oh and don't forget to wear your PDF until you are absolutely sure!! We had two guys get killed on a bass boat, the motor flew off the transom and the cable held it and the motor slammed in between their two seats, well you get the Idea, messed them both up, the weight of the motor and the speed it came crassing down on the two of them I know you say "it will be OK" but at least wear your Pdf until your sure!

 

[jay_merrill]

Re: guess how fast it will go now?!?!

Oh and don't forget to wear your PDF until you are absolutely sure!! We had two guys get killed on a bass boat, the motor flew off the transom and the cable held it and the motor slammed in between their two seats, well you get the Idea, messed them both up, the weight of the motor and the speed it came crassing down on the two of them I know you say "it will be OK" but at least wear your Pdf until your sure!

Wear your PFD all of the time anyway!

This is a good post, because most people think that an engine will just fall off the back of the boat if the transom fails. It doesn't. If the motor is under power, it typically ends up in the boat - with very predictable results.

 

[airdvr1227]

Re: guess how fast it will go now?!?!

C'mon...let the guy have his fun. Darwin would love it