Down sizing on outboard, how low can I go?

[khc1970]

I've a Cormoran 610, a planning hull Jeaneave lookalike, which I use for fishing. It's 630kg net, a mercury 100 2+2 weighing about 140kg and say 200kg extra onboard.Totalling about 1100kg at the most.<br /><br />At the moment I can achieve 32k on full throttle. The 2+2 is a little old, nothing wrong with it, just it's quite thirsty on gas. I'm thinking about changing down to a newer outboard in the near future.<br /><br />I've been very impressed with the literature on the Tohatsu TLDI, price, fuel economy, etc. But just how low can I go? I don't expect to do very high speed, low to mid 20's is about all I would expected to do. So is the 70 any good, or I'll need the 90? Is there a chart/formula I can use to work this out? <br /><br />Thanks in advance.

 

[Solittle]

Re: Down sizing on outboard, how low can I go?

khc - You can make your own formula. Lets get crazy & see what happens.<br /><br />Say you have an 18' boat with a 200 hp Yammy on it. It will burn about 20 gal per hour at WOT. If you run like most you will run 50 hours a year. That gives you 1,000 gallons per year. With fuel at $2/gal that gives you $2,000 in fuel cost for the year.<br /><br />Now we swap out the 200 hp for a 100 hp. It will burn about 10 gal per hour at WOT. Again at 50 hours per year that gives you 500 gallons for the year with the 100. With $2/gal fuel the cost per year will be $1,000 or a savings of $1,000 over the 200. Not bad.<br /><br />But wait - If the 200 is paid for you still have to buy the 100. Iboat has new 115s at around $6.5k. You need to offset this by what you get in trade for your 200. Take this amount and divide by the $1,000 in annual fuel savings and that will give you the number of years just to break even.<br /><br />You can play with the numbers & conversion.<br /><br />Have fun.

 

[roscoe]

Re: Down sizing on outboard, how low can I go?

Many experienced boaters would recommend powering your boat with 80% of the hp rating of the boat.<br />So if it is rated for 100hp, an 80hp motor should satisfy most boaters. You might be ok with a 70hp if the boat planes easily.<br /><br />I did find this on a marina website:<br /><br />Cormoran 610 - Versatile High Performance Sportsfisher with a robust pedigree. Economical and practical boat for under £10,000. e.g 32 Knots with 75hp 4stroke. <br /><br />That seems a little optomistic, given the net weight of the rig.

 

[WillyBWright]

Re: Down sizing on outboard, how low can I go?

Look at the class of motor. A 75 from a larger line would have more power. Say you had to choose from Brand A that had a 60/70/75Hp line-up and Brand B had a 75/90/100Hp line-up, I'd choose Brand B in your situation. It's a de-tuned 100 wheras Brand A's is a max-tuned 60.

 

[ED21]

Re: Down sizing on outboard, how low can I go?

I think the price of gas in England is at least double the $2.00 quoted. <br />KCH1970 is painfully aware of that though.

 

[Jdeagro]

Re: Down sizing on outboard, how low can I go?

khc;<br /><br />You got some good advice here, and how much you use your boat will help determine the practicality of changing engines.<br /><br />One thing the others did not to pick up on is that in the UK your fuel is more than 2 times our USA cost per gallon.<br /><br />I would still think that reliability is an important factor. If you feel the current engine is reliable, then I would agree with some of the others and suggest you run the numbers.<br /><br />There are a few things you can do to improve your economy with the existing motor. First determine the most used cruising speed, and make sure the boat is balanced properly at this speed. Second remove as much weight as possible (such as equipment that you do not need). Third, make sure your prop is correct for the application. Make sure the motor is trimmed correctly ( you may not have electric trim). If the running attitude is too far down in the front or too far up, concider balance and other devices you may have on the boat such as a hydrofoil. If you are using a hydrofoil make sure it is not forcing the bow down at cruising speeds. <br /><br />Hull efficiency is as important to boat performance and economy as the motor.

 

[jshoes52]

Re: Down sizing on outboard, how low can I go?

Rule of thumb is 1 hp to every 40# minimum, and 1 to every 25# optimum. I don't know from your reported weights how to convert from kg to lbs, so you'll have to do the math. I have the 90 Tohatsu tldi and love it. Same block as the 70, the only difference is the ecu which gives the 90 more top end speed.

 

[rabidfish]

Re: Down sizing on outboard, how low can I go?

And: if the weight is the same, take the 90 over the 70.<br /><br />Industry feedback all says the single most prominent complaint is "not enough power".<br /><br />I like the 75-80% rule for a minimum.

 

[rodbolt]

Re: Down sizing on outboard, how low can I go?

and sometime the smaller motor has to turn faster so the fuel savings is negligable on some applications. I see 4.3 volvo packages in 25 sou-westers and parkers here all the time. they have to turn 39-4200 all the time. same hull set up with a 5.7 turns 3 to 3400 for the same speed

 

[ziemann]

Re: Down sizing on outboard, how low can I go?

I second the vote on going with the Nissan/ Tohatsu 90 TLDI over the 70. How often do you ever hear someone say, "I have way too much power". Altho it can happen, you're not in that position with your boat and the engines you are looking at. Drop in to the Nissan/ Tohatsu repair board and take a look at the positive posts on the TLDI's. There are alot of great engines out there- and Nissan/ Tohatsu is among them.

 

[khc1970]

Re: Down sizing on outboard, how low can I go?

At the moment unleaded petrol is 85p a litre, just under £4 a gallon, with the exchange rate, about $5.75. A normal day fishing, will cost me about £40 of petrol, hence the reason I want better economy.<br /><br />I think I'm pretty well sold on the tldi 90, except I will have to save a bit more as it's £5 1/2k over here. <br /><br />Just out of interest, I read it somewhere else on the forum, the 70 and 90 are identical in everyway except, cpu and gearings. So am I correct at saying there pick up, low and mid range are also the same, only the top speed is more on the 90?<br /><br />Thanks everyone.

 

[mjbrueck]

Re: Down sizing on outboard, how low can I go?

Your fuel mileage for the 90 will be nearly identical to the 70, until you start using more than 70 hp. The 90 will use a little more gas accelerating, but at a constant speed that either engine can attain, they'll be about the same.<br /><br />If for example, the 70 will give you a top end of 25 knots, you'll get the same fuel mileage at 20 knots with either the 70 or 90. The 90 won't use more fuel until you start exceeding 70 hp (i.e. go faster than 25 knots).<br /><br />You also asked what kind of performance you can expect with different engines. Do a search for "Crouch's Formula" and you can use that to predict your new top speed with different engines. First solve for the boat constant using your current engine hp, top end and weight. Then you can use different hp's to figure out your new top end.

 

[khc1970]

Re: Down sizing on outboard, how low can I go?

Searched via google, the only formula I can find was as this<br /><br />Crouch's Formula<br /><br /> V = C/((DISP/HP)**.5)<br /><br />Where V = boat speed in knots (1 knot=1.15 mph)<br /> C = Constant (depends on boat type)<br /> DISP = Displacement (pounds)<br /> Note that boat manufacturers usually give<br /> innacurate numbers for displacement, <br /> typically on the low side<br /> HP = Horsepower available at the propeller<br /><br />In the formula does **.5 means the power of 0.5 of is it a mis-typed of *0.5?

 

[Guest]

Re: Down sizing on outboard, how low can I go?

KHC: You should also consider the e-tec. <br /><br />It is available as a 75 or a 90. But I hear the 75 is almost as powerful as the 90. I believe it puts out 82 hp. <br /><br />Just a thought. Bill

 

[khc1970]

Re: Down sizing on outboard, how low can I go?

I've also considered the etec, I know it's probably a better engine than the tohatsu, but I had a Evinrude years ago and it wasn't a very good engine, maybe due to it's previous owners' negleat. Plus the 90 etec is £900 more than the 90 tldi. <br /><br />Is it true the etec doesn't need a service till 300 hours/3 years? Anyone know what the service interval for the TLDI?

 

[mjbrueck]

Re: Down sizing on outboard, how low can I go?

Sorry, I was too lazy to look up the formula. It is the square root. From your first post (100hp, 1100kg =2420lbs, and 32knots), it gives you a hull constant of 157. So a 90hp should give you a top speed of 30 knots and a 70hp should give you a top speed of 27 knots.

 

[Jdeagro]

Re: Down sizing on outboard, how low can I go?

khc;<br /><br />Regardless of the motor you choose, the hull efficiency will make a huge difference in performance and economy. The single best thing you can do to improve both is to add trim tabs. We sell a lot of our Smart Tabs in Europe simply because of the lower powered engines and fuel cost. We have two distributors in the UK if you are interested. (www.smarttabs.net)

 

[jshoes52]

Re: Down sizing on outboard, how low can I go?

I wouldn't agree an e-tec is a better engine than a Tohatsu. Remember, e-tec 90's have only been on the market a year, and Tohatsu is legendary for their reliability. Another factor you already mentioned is bang for the buck. I priced a 90 e-tec and found it to be over $2500 more than a Tohatsu 90 TLDI when you add up all the cables, controls and installation.<br /><br />You can go to www.tohatsu.com and download the owner's manual for a 90tldi and read for yourself the maintenance schedule. They say a water pump at 100 hrs and fuel filter, air filter and drive belt at 200 hrs.

 

[JasonJ]

Re: Down sizing on outboard, how low can I go?

In my opinion, spending a ton of cash to save a bit of cash is not the best reason to replace the motor. If you had concerns about reliability, it lacked power, AND you wanted increased economy, that would be a good reason. If you are happy with everything about the motor except economy, then spending several thousand dollars to save a few dollars in gas is not wise.<br /><br />I have been seeing people running out and getting rid of their perfectly good paid off cars and trucks just to pay for a more fuel efficient smaller vehicle, but spending thousands of dollars. That money can buy an awful lot of gas. In your case, you are wanting to replace a good running motor with a smaller, less powerful motor. You will end up running that smaller motor wide open to try to go the same speeds or similar, and your fuel savings will not be that great. You will then end up with a slower boat and a giant hole in your wallet. If it were me, I would look at increasing the efficiency of the current setup by looking at load, setup, trim tabs, prop. If you have exhausted all of that and still need more efficiency, then maybe repowering can be an option. If you just feel like buying a new motor, then go for it, but I don't think you will be happy with an underpowered boat. Good luck with it...

 

[gss036]

Re: Down sizing on outboard, how low can I go?

I have to agree with Jason on this one. I run a 1989 Merc 200 HP and it too is hungry for more gas. After pricing all the new engines, I have decided that I will run and maintain my Merc until is blows up or what ever. I just read an article this AM in the paper that gas is expected to rise another 25 cents a gallon this week. That will take us to $2.20 (@ Costco) or better here in WA. So fuel is getting to be an issue at 12+ gallons an hour cruising at 42-4300 RPM.