Top speed of 2011 Honda BF 15 on 14' lund boat

tgeseron

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Apr 16, 2011
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2
A question for all the experts here!

I have a brand new 2011 honda bf 15 long shaft motor mounted on a very old (not sure of exact model) lund 14' aluminum boat. The boat is probably late 60's early 70s, so is quite narrow and light.

The boat is a short shaft but I bought a jack plate and raised up the motor so the anti cav plate is level with the bottom of the boat. It does not spray water when at WOT like I would have expected if it was mounted too low. The jack plate does offset the motor back 3 inches (mini jack plate from cabelas)

My problem is top speed, with 2 guys (200lbs each) and a large deep cycle battery, fuel, gear etc. top speed in calm water is 15mph. I did not think that was too bad but it seems like it should be faster. I tested with all gear but just myself in the boat and it planed properly (raises front end up then sits down vs just sitting down with the 2 ppl) and I still only ran at 15mph, confirmed with 2 gps devices. Am I looking at a prop change or something else? It just seems slow to me......

Thanks,

Travis
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
Re: Top speed of 2011 Honda BF 15 on 14' lund boat

When you took 200# out of that boat, things should have changed drastically with a 14' alum boat and 15 hp engine. 15 mph is about planing speed and with your rig, I would expect you to get closer to 20. Alone, 22-25 would seem appropriate.

If you don't have a tach, listen to the engine. Most modern engines will run 5 to 5.5k and at those rpm's the engine sings. Throttle response is crisp, i.e. you can be at WOT, cut the throttle back somewhat and before the boat settles in the water goose it. If it recovers performance, you are good to go. If not, you have something wrong.

Props on those smaller motors are not that critical. BTDT You can move around a few inches of pitch and do little to/for performance.

I guess your starting point is to get a tach and see where you are with respect to max engine operating rpms at WOT. Do that and come back for more help.

Mark
 

tgeseron

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Apr 16, 2011
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Re: Top speed of 2011 Honda BF 15 on 14' lund boat

Thanks for the reply! I am thinking of installing a tach on the boat to try to troubleshoot this, as I will have no clue how the engine should sound unfortunately :(

This tiny tach I have heard about seems pretty easy to use, wrap around spark plug wire and run the boat... sound about right?

Thanks again,

Travis
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
Re: Top speed of 2011 Honda BF 15 on 14' lund boat

I have one on my BS lawn mower engine. It has a single wire that wraps around a spark plug wire half a dozen times and a small digital read out. I didn't look but it probably uses 2 other wires, bringing in 12v power and ground to operate it. Very reliable readings from it.

Even though the BS uses a magneto ignition fired by a magnet in the flywheel passing by a coil it makes you realize that on a 4 cycle magneto ignition engine, every time the flywheel passes the magnet the plug fires. Well on a 2 cycle that's ok because you have combustion every rotation. On a 4 cycle, you combust on one rev and clean out and load up on the next then repeat the process. Really doesn't matter that the plug fires during the clean out cycle.

If your Honda fires every rev then your tach will record accurately. If not, it will read half what your engine is really running.

To test, hook it to your plug and whatever else it needs. Crank your engine and let it idle. Idle speeds are on the order of 650-700 rpm's in the water, in gear which probably relates to 900-1000 rpms in N. If it reads about that you are good to go. If it reads half that then multiply by 2 whatever your reading is for the correct number.

It will probably read right as the simplest form of ignition is a magneto; magnet on flywheel passing a coil of wire and that will happen every rev so your plugs on your 4 stroke honda probably fire every rev, even though only every other rev produces power.
 

John_S

Rear Admiral
Joined
Jun 21, 2004
Messages
4,269
Re: Top speed of 2011 Honda BF 15 on 14' lund boat

An alternative to Tiny Tach: Trail Tech or Works Connection

I picked up a one wire digital tach/hour meter last winter, when I was working on a new to me vintage snowmobile. It works by picking up the pulse in the high tension lead. You just wrap the wire around, and a little electrical tape to hold. Plans included though, using on my small outboards, not just the snowmobile.

Earlier, I ran this tach on my 25hp 2 stroke Merc, and over the weekend on my Nissan 40C. While the Merc fires on every stroke, the Nissan fires at the top and bottom of each stroke. The tach was configurable and handled both. Also has 4 stroke setting for one pulse for 2 revolutions.

It also has a fast/slow update mode. Slow helps for idle speed adjustments where the digital number can jump around. It also has a noise hi/lo feature, which I haven't needed to use.

The tach can be found in the link below. I purchased mine, new, off ebay for around $32 w/shipping. So far it has been a great multi-engine tach, and thought it would be of interest to others.

http://trailtech.net/tto_tach-hour.html
 
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