Repower up from 60hp Johnson/16' Magnum Boat

[Flipv1983]

Hey guys, I posted the other day and am fishing for more info regarding powering up my boat. My buddy and I took the women out today for a day of fishing. Here's my issue and question. I have a 1985 60hp Johnson (2 cylinder) on a 1988 16' Magnum boat. The max HP rating is 90hp and the boat comes in around 700lbs dry. With myself and another person on board the boat takes about 20 seconds to plane. With the 4 of us it didn't plane one time the whole morning. I'm looking at buying a 1988 70hp Johnson from my mechanic. So that's 10hp and another cylinder. What kind of difference, "hypothetically speaking", should I see if any? And I understand that's a lot of weight but on days we decide to take the girls out I'd like the performance also. Is 70hp sufficient? I don't wanna jump past a 3 cylinder motor. Thanks, Phillip!!

 

[bonz_d]

Re: Repower up from 60hp Johnson/16' Magnum Boat

Don't know if you'll see a whole lot of improvement accept in maybe the holeshot. Going to the 70hp 3 cylinder is going to add almost 100 lbs over the 60hp.

I s the engine coming up to operating RPM when you have that load in it? Maybe a prop change would be a better solution. Bigger diameter, smaller pitch.

 

[JB]

Re: Repower up from 60hp Johnson/16' Magnum Boat

The 60 is less than 70% of the max rated HP. Rule of thumb is use at least 70% of the rated max for acceptable performance.

A 70 would meet that standard, but might need a better prop as well. Is the 70 you contemplate equipped with PT&T?

 

[Silvertip]

Re: Repower up from 60hp Johnson/16' Magnum Boat

Although this is a Johnson/Evinrude forum you might consider a 75 HP Merc three cylinder. These are large displacement engines with big gearcases and are good engines. The 90 HP is the same block. Going to the 75 - 90 Johnson/Evinrudes adds more weight although they are stump pullers and would work well with a heavily loaded boat.

 

[bonz_d]

Re: Repower up from 60hp Johnson/16' Magnum Boat

OK a 60hp would only be 3 hp under the 70%. Do you really feel you would see any performance change from a 60hp to a 63hp?

Would you really also gain much by adding 10hp and another 100 lbs of weight? I think the only advatage would be the added torque of the extra cylinder under accelleration to plaining speed. Which a prop change could also achieve for a lot less money.

 

[Flipv1983]

Re: Repower up from 60hp Johnson/16' Magnum Boat

Rpm range depends on load. With everyone on the boat the other day, no, the boat stayed in the mid 4000 range as it was operating like a barge! She just doesn't have the power to get the boat up w that load. I already tried the prop change route. Under a load consisting of myself and 12 gallons of gas. I went from a 12x17 which had me running 30mph (GPS) @ 5120rpm WOT and taking me about 20sec to plane to a 12.25x15 which had me running 28mph (GPS) @ 5420rpm WOT and getting me on plane round 15sec. After talking to the guy from Prop Gods down in Sarasota I decided to lift the engine one hole which brought me up to 29mph (GPS) @ 5490rpm. Hole shot improved a hair but not significantly. Put another 200lb man on the boat or a couple of women and were back up to a 20sec plane time if I run w current.

Another factor I'm taking into consideration is hull design. I know it takes more motor/hp to get a V bottom on plane. This 60hp came off pur 1st boat whih was an 1120lb 17' Trihull Skiff and I had better luck getting that big Bertha on plane w heavy load.

Yes the 1989 70hp Johnson I'm looking at picking up off my mechanic has PT&T, its the Tracker model.

 

[fender230]

Re: Repower up from 60hp Johnson/16' Magnum Boat

Have you tried doel fins? They did wonders for my 83 Evinrude 60hp 2 cylinder. I plane almost immediately; even with a full boat. I agree with previous posters that adding 10HP may be negated by the extra weight of the 3 cylinder.

 

[Flipv1983]

Re: Repower up from 60hp Johnson/16' Magnum Boat

Yes, I have a Sport SE on the motor now. It definitely helped under light load. What size boat and load are you pushing? Fiberglass or aluminum?

The difference in weight between the two engines is 70lbs. Dry weight on each is 60hp weighs 187 and the 70hp weighs 257. I feel that between the displacement and different gearing the weight may not be so much of an issue.

Silvertip: I was looking into 75hp Johnson's but they seem very hard to come by in the later models before production stopped. I've come across numerous Stingers but I'm not trying to equip w a 1970s model either. Too many issues that can come up. Do you feel the 70hp won't be enough? Is the gearing that much different from the 75hp? I'm looking at the numbers right now and the displacement for the 75hp is 49.7 which is the same as te 60hp and the displacement on the 70hp is 56.1, the 70hp actually sounds like a bigger engine as the weight is also heavier then the 75hp.

I contemplated going to 75 Yamaha but my mechanic is amazing and has always kept my service costs very reasonable. He only deals w OMC. Plus if I get his motor off him if I have any issues w anything he does to it before selling to me he'll cover, like a warranty.