hi thrust outboards

Greetings all. I have posted before about my Fjord 27 and scrapping the inboard (old, feeble and reputedly self destructing at higher revs) and putting on an outboard bracket (flotation). I hope to solve the engine replacement and also lack of auxiliary at the same time. I don't want to get into the inboard/outboard arguments here but in brief, I have done and will be doing loads of work on her as a project as much as anything. The aft cabin will be going, the water tank and batts will be shifted forward and I will put in two new fuel tanks as far forward as I can get them. Assuming I won't be dissuaded from this, my question here is one of most suitable outboards.

The current lump is 1000lbs of Volvo supposedly putting out 106hp. It protests if i try to open her up a bit. The boat is a semi-dis hull and I have seen video of them getting pretty much on to plane and licking along quite well. Although I would mainly be at low speed on rivers, I want to be able to wind her up when coastal
As she is a heavy, well built boat, I thought that 2 x 60hp Yam Hi thrust outboards would be an interesting set up. I could run them together at lowish revs on the river or even run one at a time with the other tilted up. (is this possible steering linkage wise??). If I wanted some oomph when coastal, what sort of speed would two 60 hi thrusts be able to reach??

My other thought was perhaps twin Tohatsu 90s or a big single outboard and something like a hithrust 25 as a kicker. The impression I have got through my reading is that apart from the Bigfoots and Hithrusts, outboards tend to deliver speed rather than shoving power and I was hoping that my intended set up would give me the best of both??

As far as cost goes, my browsing suggests that something like a 150-200hp single will be dearer to buy than 2 x 60hp hi thrusts which are also far cheaper than a new diesel inboard. I won't do the miles to warrant worrying too much about longevity of the diesel lump.

Your thoughts would be most welcome

arthor

 

Very convoluted, too many variables.

What is your question?

Why do you want "Hi thrust outboards", what's wrong with regular outboards?

Bigger diameter, larger pitch, lower RPM is going to give you higher thrust but what for?

Outboards are designed as is. You can mess with the props for your particular application.

If you want hi thrust then you want a big diesel, low RPM, large diameter prop.

It sounds too me like you may already know the answers you're looking for.

Pictures?

-Tom

 

you must realise that two 60 hp does not equal a 120 hp ..you will not get the speed of a 120 hp from 2 x 60s and usually you have to increase the prop pitch by 2 inches from a single 60 hp set up ....running with one is possible (required to be able to plane on 1 outboard in south africa) ....personally I would just stick a 115 hp on it and a kicker unless you can get some specifics from someone who has done it ..

 

The reason for the high thrust model is that he has a semi displ boat and that will give the best result in that speed range with it's higher gear ratio and larger prop diameter. I also agree that a single and a kicker is favored if some redundancy is wanted. Steering twins with one tilted is not a problem though. Just have to get the geometry right. Although the kicker can be arranged to steer from the main engine, it is also possible to run the kicker straight and use the main as a rudder. Works better than you might think although not the best for fuel economy.

 

many thanks for the replies.
When I have mentioned putting a pair of outboards on a semi displacement boat before, many have mentioned that they are aimed at speed rather than low end grunt. I just thought that perhaps hi thrust/bigfoots might be more suitable for a semi rather than a full planing boat. I do take the point that 2 x 60 does not equal 120.
The Fjord 27 has had 200hp diesels fitted but these are generally heavy TAMD Volvos. I have priced up some of the newer diesels 175-200hp and they are horribly expensive. My initial thoughts were to go with a pair of Tohatsu 90s, fairly lightweight and actually cheaper than the diesel. I would also have to solve the aux problem if I got a new diesel. Plus, I want to lower the pilothouse floor.
All ends up, I feel that outboard power is a good way to go. My question now is do I go with
a)A twin set up (90/115s) and get some chunky props.
b)Something like a 175-200 single with a 25 hi thrust kicker.

Many thanks again for your help

 

These are trully the Hi Thrust outboards. http://www.yamaha-motor.com/outboard/products/subcatspecs/4/specs.aspx
I've seen the 9.9 push a very heavy fishing boat some 30' in length up river on the Oregon coast with about a 5 mph current.

 

what speed do you want to acheive? work out the prop pitch at 3000 rpm neccessary to give this speed at 30% slip and then find the engine that will turn that prop....

 

Answer: b)

Thanks for clarifying.

-Tom

 

I have been over and over many permutations and for a while was thinking about a new main and using a hi thrust 9.9 as a spare. It could also be used for slow speed manouvring and bimbling up and down the river.
How do you think it would serve as a spare if I were 10 miles + out to sea?
It also has the advantage of being light enough to go on a stock outboard bracket. If a 9.9 was sufficiently powerful enough to use as a spare in most conditions, I would go with that and something like a 175 hp outboard as a main.
As far as speed goes, on the river, it would be 5kts max, well within hull speed. I do want to venture out further in it and these boats are good for 15-20kts but obviously they need the power and the fuel burn would be quite heavy which is why it would be an occasional thing. This is why I don't want to put in £15 grands worth of new diesel when most of the time I will be going at hull speed.

If the general concensus is that a Yam 9.9 hi thrust will serve as a spare, I can get one of those to add to my current set up to see how she goes. If she is good as a spare and moves the boat along quite happily then I will be happier leaving the existing lump in until she goes completely. Then it is a main engine decision further down the line rather than trying to make two decisions at once. Does this make sense??

So gents and ladies, we have arrived at a much simpler question.

Will a Yamaha 9.9 Hi Thrust push a 27'Selco along at hull speed in river/estuary or even coastal waters even when the current/wind/tide is adverse?

I love this forum. Just asking a question gets so much good stuff back which really helps to focus the mind as to what the original question was really about.

thanks again

 

"Will a Yamaha 9.9 Hi Thrust push a 27'Selco along at hull speed in river/estuary or even coastal waters even when the current/wind/tide is adverse?"

I'll bet she'll make 6 knots in calm seas. "Adverse current, wind, tide" is a whole different scenario...

-Tom

 

I've seen several off shore fishing boats off the Oregon coast with the 9.9 as a kicker engine. They were all inboards with a lot of stuff hanging down and the two I talked to were happy with the engine. Lots of wind in Oregon and all rivers have strong current. Call your dealer and see how many they have sold and to whom. Then call the to whom and get their advise. You may need to go one size up TO 25HP(2O3 LBS.), but I doubt it.

 

Thats about what I tend to see and read regarding the kicker. 9.9 is too small according to some and fine according to others.
I have spoken to a Mercury dealer in Scarborough (the UK one on the North Sea) and he thinks that as an aux, a normal 9.9 is fine, a Bigfoot even more so. He cites many boats at Scarborough who head off into the North Sea with even a 6hp as an aux. Purely as the difference between limping home and calling out the RNLI. It won't push me up the Humber against the tide but it is doubtful whether a Hi thrust 25 would either.
Bigfoot 9.9 - 85lbs/£2600
Yamaha 25 Hi thrust - 189lbs/£5169
The Bigfoot is light enough to go on my existing bracket and I can sort out the main decision later.

Many thanks for your input people. Very much appreciated.

arthor

 

Pistnbroke:
Why would 2 60 hp OBs not match one 120?
Sounds like old wifes tales to me.

Easy Rider

 

Arthor,
The high thrust OB your'e looking for may not be labeled "high thrust". I have a 40 hp Evinrude that has a gear ratio of 2.67 to 1. I think it takes almost a 14" propeller. Most OBs are much higher geared as in 1.8 to 2 to 1. Also if you find 2 OBs that are 2.4-1 one may have clearance for a propeller 1/2" more in diameter. OBs are made w high gear ratios and small propellers to accommodate the wide variety of boats they typically go on. Most people that operate OBs don't understand that one can over speed, damage or self destruct an OB. AND the "high thrust" OBs are designed to push a big boat when trolling for fish or getting home with a main engine failure. So they aren't suitable prime movers either. The designer has much more flexibility choosing propeller diameters and loading with inboard boats that w OBs. To get closer I'd half to see pics of your boats hull (especially aft) and some specs.

Easy Rider

 

Where did you get the idea that high thrust outboards are not suitable as prime movers ???? As long as at WOT the prop loads the motor down to its max rpm what is the problem ????