RPMs with Hardline tach/hour Tohatsu MFS15C.. yes another.. RPM/prop question kind of

[LudwigO]

Ok, I might plunge into this as well..

I bought a Hardline tach/hour meter after reading alot about it here regarding WOT rpms and propellers. So, I mounted it on my MFS15C and tested the different modes regarding 1P1R, 1P2R, 2P1R and found the 2P1R to show the correct idling rpm at about 900. All fine so far. By the way the boat is a small Askeladden 14-2210 14 feet boat (180kg empty) with max 15hp rating. With family, gas, motor, beer etc we end up at about 440kg in total including the boat.

But when running WOT I only get 2740rpm when cruising at 16,6 knots (gps measured). I have the stock 10p propeller which gives a theoretical speed at 2740 prm of 10,5 knots? There must be something fishy here...

When doubling the figures to 5480rpm I get the theoretical speed of 21 knots. When taking the load and slip into calculation the 16,6 knots is fairly reasonable.

Can anyone explain this rpm problem? I have tried the other modes and the values shown are way above reasonable figures. I have also tried a lot of different routings of the induction cable. I dont think its due to the wasted spark either. In fact the tach works fine on my Kawasaki 15 hp OHV on my lawnmower, so..

I am thinking about switching to a lower 8 or 9 pitch prop in order to get better hole shot, since the top speed will probably not suffer that much from such a switch I think. I have tried all different trim angles by the way.


Hmm this got long too.. :watermelon:

 

[pvanv]

Re: RPMs with Hardline tach/hour Tohatsu MFS15C.. yes another.. RPM/prop question kin

You have a middle-to-heavy load, so less pitch may help. Slip calculations are a guess at best. Theoretical speed is also just a guess. Each boat is unique. Your motor has a WOT electronic speed governor (ESG) which will cause stuttered ignition to prevent going much above 6,000 RPM. Is your motor too deep in the water? The antiventilation plate should be near the bottom of the hull.

 

[LudwigO]

Re: RPMs with Hardline tach/hour Tohatsu MFS15C.. yes another.. RPM/prop question kin

Thanks for helping out!

The anti-vent plate is near the bottom of the hull or in line with it. The upper plate is above water at plane so my interpretion is that it is positioned corretcly. My reason for calculating the theoretical speed is to see whether the RPM reading at WOT was reasonable or not.

When doubling the figures it made sense, but did not explain why it read right at idle? I also think that 2740 RPM at WOT seems very loooow so the reading must be off or the engine is really struggling, but it doesnt sound like it is. I am absolutely sure that the ESG is not kicking in. Been up there on race tracks with cars so I know that feeling..

If the reading is correct, then the theoretical speed is over 30 knots (not likely) and not supported by theoretical calculations.

Should I go to 9p or directly to 8p if changing the pitch, original propellers presumed?

Why cant this just be easy? :facepalm:

 

[Jon Boat Jim]

Re: RPMs with Hardline tach/hour Tohatsu MFS15C.. yes another.. RPM/prop question kin

The rpms at WOT should be 5000-6000 according to Nissan specs. Your data at 5,480 rpm seems very reasonable to me for a 15HP motor.

The Hardline Tach on my two cycle Nissan 18 HP jumps around a lot, so I have to guess what the average is.

 

[Sea Rider]

Re: RPMs with Hardline tach/hour Tohatsu MFS15C.. yes another.. RPM/prop question kin

Do you have back transom water splashes: over or out transom ? if so, you have tail drag issue which lessens your rpm a bit. Have you wrapped 4 turn of the pick up cable tight and secured it with tie wrap provided as per instructions ? Which is your wot reading with you alone on flat, calm waters ? With 5480 you're right in the midle between min, max wot parameters.

Will achieve + 150/200 revs switching to a 9 pitch. If you don't have any legal issues pass that horse to 20 as you intend to do, will perform much better family oriented, if overpowering, you can always throttle less...

Induction tachs on 4 strokes engines have more steady & accurate readings at idle/wot than 2 strokes same model engines, which shows erratic readings at idle and steady/accurate readings passing fast idle/wot.

Happy Boating

 

[LudwigO]

Re: RPMs with Hardline tach/hour Tohatsu MFS15C.. yes another.. RPM/prop question kin

Thanks for all replies! This forum is another great piece of gold found on the net!

Anyhow.. I took the boat out this evening for a spin on my own to experiment with the meter/trim etc in order to see if I could get it right. Nice calm water. I have also been in contact with Hardline to hear their theory. They did not have a good one unfortunately.

First attempt exactly by the instructions, 4 tight loops around the cable. Results were strange, about 3x the correct measure at idle with 2p1r setting. Motor set to idle at 900rpm from factory. Good to know! :joyous:

Second attempt, one tight winding still 2p1r, still as bad.

Third attempt, just taped the cable on the ignition cable parallel about one cm. That showed the correct idling, but still just half the full throttle?

So I just put the meter into 1p1r mode, did not care about the double idle and got the presumed correct WOT value 5975 rpm which is almost at max (6000rpm). The 5975 moved the boat at 20,6 knots (gps measured). Not that bad I think.

The water level when at plane is about 5cm (2in) above the anti-vent plate which i guess is rather ok? The only water splashing there is going sidways/back from the transom.

Now jacking it up to 20HP will not be a legal issue, just money and possible warranty I guess. If I do, I will try to get the carb from some other country in order to cut the price a bit..

 

[Sea Rider]

Re: RPMs with Hardline tach/hour Tohatsu MFS15C.. yes another.. RPM/prop question kin

Is it possible to try someone else same tach to compare readings ? Could have a faulty tach. Wrap 4 turns again and keep tach set to the setting you achieved 5975 rpm. Is the posted price stated in US dollars ? if so, you could cut that huge amount by half or much less if importing direct.

If you only have small side water splashes at back transom that engine is riding happily well transom seated. Achieved 20.6 knots with a 15 is a nice top speed, definitely planed much faster with you alone than when tested with family.

Happy Boating

 

[LudwigO]

Re: RPMs with Hardline tach/hour Tohatsu MFS15C.. yes another.. RPM/prop question kin

Thanks for the input!

I did a calculation for "theoretical speed" with prop RPM @ 5975 engine RPM which would give 22,9knots. So I think that 20,6 is, as you say, is a nice top speed for a 15. No doubt about that!

I dont have the possibility to try another, however Hardline will take my unit back for exchange. Since I live in Sweden, the shipping of the unit to the US will be hefty.. so I'll have to think about the economical side there.

Regarding prices of carbs, yes the price i stated is translated from SEK into US dollars. Now, Ill just have to find some good dealer that sells to Sweden over the net.