Yamaha 4 Stroke Question

[shorty28]

I'm in the process of buying an aluminum jon boat that I would like to power with a 60HP Yamaha 4-stroke. The boat is 18 foot long, has a 60" bottom, and a 82" Beam. The .100 thick hull weighs in around 1000lbs without the motor. It will usually have 2 passengers, 15 gallons of gas, 3 batteries, and and minimal fishing gear. I chose this motor based on weight, and my local service. If I upgrade to a 75hp or 90hp the weight jumps from 240lbs to 370lbs. With the gas, batteries, and passengers near the rear of the boat, I worry about navigating in shallows, as well as porpoiseing (sp?) if I go ahead and get the bigger motor. I'm also content with 30mph, which I'm guessing it can do based on the performance tests on Yamaha's website. My question is this:<br /><br />1. Does anyone have a similar set-up that can comment on the performance.<br /><br />2. Should I consider one of the high thrust models, even though this isn't a pontoon boat? It is a modified V hull (12degree deadrise). Not sure if this boat fits in the displacement category or the planing category.<br /><br />Thanks for any input!!

 

[TOHATSU GURU]

Re: Yamaha 4 Stroke Question

You are underpowered with a 60 hp.

 

[Silvertip]

Re: Yamaha 4 Stroke Question

I agree. Check the plate on the inside of the boat for the maximum rated HP. You want to be at or very close to that rating.

 

[shorty28]

Re: Yamaha 4 Stroke Question

I was kinda concerned with that. Its rated for a 115. I'm just not willing to go over the 60HP on a 4-stroke. Those motors are gigantic and heavy! For my taste anyway, I plan to do some duck hunting in marshes, fish shallow streams etc... I think I'll go back to looking at the 90hp 2-stroke yammy, might be my safest choice....or maybe a smaller boat. Thanks for the input!!

 

[Silvertip]

Re: Yamaha 4 Stroke Question

60 HP is just a tad over 50% of rated max HP. I can assure you that from experience you will not be happy with the performance and you will not be doing the engine any favors either. Why not look at either a 75 or 90 Evinrude E-tec or Merc Optimax two strokes. Better performance, same or better economy, nearly as quiet a little lighter and less maintenance, no oil/filter changes, no valve adjustments, and less moving parts.

 

[JB]

Re: Yamaha 4 Stroke Question

If what you have is really a Jon boat (flat bottom) and weighs around 1800lb as launched I calculate a WOT speed of about 40-45 with a 60HP engine and optimum prop.<br /><br />I ordinarily recommend 70% or more of rated HP for satisfactory performance, but Jons are different with their flat bottoms full length and can do well with 50% of rated HP.<br /><br />The same formula gives me 55mph with a 115. That would be pretty miserable in a Jon if there is anything bigger than a ripple on the water.<br /><br />If you would be truly happy with 30mph and wouldn't mind some reserve for emergency I think the 60 would serve your purposes.

 

[BillP]

Re: Yamaha 4 Stroke Question

I somewhat agree with JB. The flat bottom will let the boat perform with way less than "normal" hp quite nicely. I'm currently running a 17' boat with a "very close" to flat bottom (but narrower @ 64") on 65hp and getting 32 mph (gps)...loaded with 2 adults and fishing gear at approx 1400lbs. I propped it with a larger diameter/flatter pitch for heavy loads. It jumps (3-4 seconds)on a plane with 2 adults and gets on a plane in approx twice the time with 4 adults, 18 gals fuel, fishing gear, etc.. With 4 it still gets high 20s mph.

 

[swist]

Re: Yamaha 4 Stroke Question

One of the reasons that it is always said you should get the biggest engine under the max rating is not so much that you need that much power for normal use, but in heavy seas it is good to have a reserve you might need when things get dicey.<br /><br />I would think that jon boats and other calm-water-only vessels would need to look at this differently. At the weight/cost of today's 4-strokes and DFI 2-strokes, you may just be wasting money for HP you will never use, and could possibly even be creating a handling problem if you make the boat too stern-heavy.

 

[TOHATSU GURU]

Re: Yamaha 4 Stroke Question

There are a great many factors that can go in to choosing an engine for your boat. Regardless of brand, some important considerations are weight, budget and horsepower. While a minimum hp engine may do exactly what you may like in terms of top end speed, weight and budget it is rarely your best choice. The cons of going with a smaller engine are as follows: In terms of resale going with a smaller engine means you will have a hard time unloading the boat on someone else in the future. While you may achieve your desired max speed(I really question that you will achieve 30 mph), you will have to run at wideopen to stay there. In addition you will have a hard time getting up on plane with a prop that lets you get that desired speed. Your fuel economy will be nothing you would want to tell your friends about and your going to be spending more to keep up that 4-stroke than one of the DFI 2-strokes or a carburetored 2-stoke. The pros for that Yamaha 60 4-stk are: It's a quality product and if you are going to do a lot of trolling it's friendly on gas. <br /><br />Other than a sailor with a blow boat auxillary engine you never hear the phrase "I am glad I got this smaller engine". What you will always hear is "I wish I had gotten the bigger engine."

 

[BillP]

Re: Yamaha 4 Stroke Question

I don't know if these numbers help but they may...my 65 is a carbed 2 stroke and I usually cruise at 23-25 mph (usually too rough to go faster). It gets 5 mpg at cruise with 2 adults/fuel/fishing gear aboard...which is outstanding mpg in the boat world. That's what the flat bottom does for you. Running near full throttle for another couple mph sucks the tank dry approx 2X as fast going the same distance.<br /><br />I didn't mention earlier but I also have a 50hp 4 stroke on a pontoon boat. It runs fine but weighs the same as my 65 hp (which is actually a commercial version of the domestic 70 OMC). There is no way I'd take my 2 stroke off and replace it with a 4 stroke of any size. Simple is better IMHO.

 

[mallardjusted]

Re: Yamaha 4 Stroke Question

It would be interesting to know what brand/model you have, and if it actually has a flat bottom or semi-v?<br /><br />I wonder because my boat is a Lowe Roughneck, modified bow-V, with: 83"beam, 60" bottom width, 1 foot shorter (17'), with the same hull and frame thickness, yet it weighs under 600 lbs dry????? Seems strange an extra foot would add 400 lbs. Then again, I don't know what kind of superstructure you have on this boat?<br /><br />Anyway, I have the 50HP yamaha 4-stroke, and I love it! It has more than enough power, and I occasionaly have 4 people and gear in the boat. I also have the extra weight of an 8hp kicker and bedliner sprayed on the boat. Of course, my HP ratings are a little less - 40/65 (tiller/remote).

 

[Guest]

Re: Yamaha 4 Stroke Question

Shorty:<br /><br />I have a Lund Alaskan 18' tiller. It weighs in at around 760# empty and no motor. I have a 50 e-tec on it and it goes just fine. I get up to about 33 mph under ideal conditions. I think you'll be fine with the 60. <br /><br />By the way, is it a fuel injected model? I was fishing in the local river on a cold day a few weeks ago and saw a guy with a 50 Yamaha (without FI). He said the motor was great except on cold day. He was having a heck of a time starting it. <br /><br />My brother has a next size up Yamaha 4 stroke. It's an excellent motor. Good luck!

 

[shorty28]

Re: Yamaha 4 Stroke Question

Wow, thanks for all the reply's. To further clarify, it is a modified V with a 12degree deadrise. I rounded up on the hull weight. I think the book says 750lbs, not sure if that includes side console, live well, floor and side skin, rod locker, etc...so I figured when all is said and done it would be a 1000lb hull, and then add batteries, gas, motor and gear. <br /><br />Thanks for the input!! I think I'm going to wait until I can get on the water with something similar before I order the boat.

 

[mallardjusted]

Re: Yamaha 4 Stroke Question

shorty28,<br /><br />So it sounds like it's really up to you and your preferences. The 60 should do it (sounds like you would only be about 300 pounds above my boat's weight), but something larger will get you there faster! If you can move weight forward (gas, gear, a battery or two), or use tabs, the added weight of the bigger motor shouldn't be a problem. Maybe it's down to you wanting to go 25 or 30mph vs 35 mpr? Or certainly there is a price difference in the decision .....