F75 vs F70 Yamaha Outboard

[jcminnesota]

I have a 16 ft Alumacraft Classic with a 2013 Yamaha F70. It runs great and gets good gas mileage. However, I am having trouble pulling my son on a slalom ski...at least deepwater start. I am running a 14 inch prop....down from the stock 15 inch for my boat to increase the pulling power. My question is whether the Yamaha 75 would do a better job with the hole shot pulling power. I understand that the engine displacement is a major factor in the initial torque of an engine....and the 75 (detuned 90) has over 33% more engine displacement in terms of cubic centimeters. The engine also weighs 100 lbs more than the 70 which is a negative factor. Does the much larger engine displacement mean that the 75 would have much more initial pulling power for skiing? I don't expect the 5 HP difference to make any difference, particularly with the extra 100 lb weight of the motor. That is probably a wash.

My dealer would charge me $2500 to trade my F70 for a new F75. Would it be worth it?

Thanks! I want to keep this boat but I also want to be able to use it for watersports, particularily slalom skiing. The primary use is for fishing..and the 70 is perfectly adequate for that.

Thanks!

 

[99yam40]

Re: F75 vs F70 Yamaha Outboard

I would be talking to some prop guys for the correct prop not changing a motor for 5 more HP

 

[Bamaman1]

Re: F75 vs F70 Yamaha Outboard

My first question is whether your boat will pull the redline with a 14 pitch prop? In other words, is it properly propped for every day running as it sits?

You might first want to invest in a new 12 pitch aluminum propeller and see how it tows before trading motors.

The 75 hp motor is in the 75-90-115 family, and you're right about it being heavier. And it's much more cubic inches. I'd be looking to go to a 90 hp if the boat's rated for 90, and if I was trading up.

I was looking on a big online boat retailer (I), and they're selling the 75 hp for $7.9K and the 70 for $7.2K--not that much difference. The 90 is $8.4K.

 

[99yam40]

Re: F75 vs F70 Yamaha Outboard

some times it is best to have seperate props for what you will be doing one for regular cruising/fishing and another for heavy loads/Pulling skiers

 

[smittylv1]

Re: F75 vs F70 Yamaha Outboard

I would have to agree with 99yam40...with the correct prop selection for the 70HP, you should be able to pull the skier right up. Maybe a 4 blade SS?...or a 3 blade pontoon style.

 

[jcminnesota]

Re: F75 vs F70 Yamaha Outboard

Thanks to all. I will look at a prop change. However, the current 14 pitch alum prop puts me about 200 rpm over the 6300 rpm limit. A 12 pitch will certainly allow over revving the motor. But if I am pulling a 200 lb load in the form of a slalom skier, would that act to reduce the rpm? If so, I could just use that prop for skiing.

A second question for the community. I spoke to the dealer about putting on a 14 pitch SS prop to reduce flexing while pulling a skier. He advised against it indicating the weight of the prop would slow initial spin up and may slow the hole shot. That doesn't sound right to me. Any comments out there?

 

[Bamaman1]

Re: F75 vs F70 Yamaha Outboard

If you went to a lower pitch prop, you'd have to carefully watch the tachometer for over-reving. Your engine is turning fast enough already.

I cannot help you on a stainless prop on a smaller horsepower motor as most stainless props are 115 hp and over.

 

[mommicked]

Re: F75 vs F70 Yamaha Outboard

I second the 4 blade suggestion. I put a Solas alum. 4 bld. on my F-90 and it totally improved my boats performance over the factory prop. My brother put one on his F-115 and it improved his performance over the factory 3 blade stainless prop.