Power and speed

[DutchMerc]

I have a small 4.5mtr.x 1.70mtr.polyester Cresent boat with a Mercury 402 2Stroke 40HP longtail motor, and i would like to know what the max. speed on this combination would be.

 

[steelespike]

Re: Power and speed

does the boat require a long shaft motor?
I think the 40 is listed for up to 33 mph on a typical boat.
Your speed will depend on the boat weight,hull design,proper setup
and prop.

 

[Silvertip]

Re: Power and speed

No data! Depends on load, weight of boat, type of hull, prop pitch, prop style and manufacturer to name just a few. 1.7 meters is a 5.5 foot boat so it should be dangerously overpowered regardless of hull type.

 

[DutchMerc]

Re: Power and speed

I put some photo's in the original post, maybe this will help?
I think the weight of the boat is ~450 Kg. and the length is 15 ft.
I took the boat out today, but i only got a max GPS-speed from ~10 KM/HR (~5 MPH) This does not seem verry good to me. I think this motor should be able to do more... What do you think about this? Is the tail to long? The prop's center is 30cm below the boat. I have no idea about the spec's for this prop. Can i measure the pitch? The diameter is 26 centimeters.

 

[SnappingTurtle]

Re: Power and speed

The boat is closer to 15ft. x 5.5ft., deep-V with a Cuddy Cabin, so I don't think it is overpowered by any means with a 40hp motor.

I think the max is, or was rated as 60hp for your boat.

I was on one yesterday very similar to yours with a new 90hp Yamaha 2stroke, and it is a very flat, tame, stable ride.

I think the long shaft is probably a little long for the hull, and will eat up some of your top speed, but would still be nice for the rivers and canals. Just watch the depth gauge.

 

[DutchMerc]

Re: Power and speed

So, if the tail is to long, this means i am loosing speed? what speed can I expect? I think there is a problem with this 40HP Mercury 402 , because i could not get more than 10 Km/hr. Does anyone know the max. rpm for this motor?

 

[Silvertip]

Re: Power and speed

Sorry -- I missed the 4.5 meter and took the model and 1.7 meter as some sort of sport/racing shell. If your estimate of boat weight is correct, that is a fairly heavy boat for 40HP and if it is rated for 60 HP maximum, 40 HP is still well below the 75% of maximum rule of thumb for power. In other words to get acceptable performance from any boat, it should not be powered with anything less than 75% of the maximum rated HP. However it should also go faster than 10 km/hr and although it would struggle, should get on plane with a light load. The length of the leg is really unimportant but the location of the antiventilation plate in relation to the bottom of the hull is important. The engine needs to be raised on its mounting so that plate is flush to an inch above the keel. With so much lower unit in the water there is a fair amount of drag. Your prop diameter appears correct (10+ inches) but you cannot measure pitch. Pitch is normally stamped somewhere on the prop -- either on the barrel between the blades or on the hub behind the prop nut. 13P would be typical for that motor. If you have no maintenance history on the engine it may be due for a tune up.

 

[SnappingTurtle]

Re: Power and speed

The crescent I was on was the newer 5m version of your boat, and I was told it weighed about 500kilos, plus gear and passengers, so I think your estimate is probably pretty close.

He was running a normal length lower unit, and I noticed when it was launched that the “anti-ventilation plates” were right at the bottom of the hull, which leads me to believe yours maybe to long. As Silvertip mentioned, this would present a lot of extra drag, thus slowing you down. How much would be a question for the experts here.

It really is a nice boat for the river & weekend trips on the water, even with the smaller motor you have. But you might want to see about trading up to a 60hp in the future. I wouldn't go over that, although the 90hp is really nice when fighting the river current & pulling a skier or tube with a full load.

 

[DutchMerc]

Re: Power and speed

Thanks guys, i see the problem here. It seems that there is a problem with my motor. Could this be just a tuning-problem? Should i take this to an expert or can i do anything myself? What speed is acceptable for this combination? Can i get 25 km/hr with this motor, or should i trade it in for a bigger one?

 

[redfury]

Re: Power and speed

You will want to make sure all the cylinders are firing. My friend just swapped out an old 100hp Johnson for a 40hp mariner. I didn't think he'd get any performance out of it at all. The 100hp shot it across the water at 35mph. This is a fairly deep hulled Sylvan 16ft runabout...no lightweight that's for sure.

27mph with the 40hp Not too quick out of the hole, but he's happy he's got a reliable motor and can use this heavy glass beast until he can find a tinny to put it on later.

When the 100hp wasn't firing on 2 cylinders, it wouldn't go over 8mph.

 

[CATransplant]

Re: Power and speed

Yes, I'm certain that your engine is not running properly. If it's only giving you 5mph. Even too long a shaft would do better than that...much better. It sounds like your engine is running on one cylinder or you have a slipping prop hub.

 

[SnappingTurtle]

Re: Power and speed

My little 6hp twin cylinder lost a sparkplug cable last week about 100meters downstream from the dock, (my fault, I changed them before I left). I swear I could swim faster than the motor would push the boat. I could put it in neutral and it would rev fine, put it in gear and it sounded weak, but ok. Crawled to the dock and stuck the plug cable back on and it ran, like before, still slow, but faster than I can swim.

I agree with the others that something is probably not 100percent with the motor, although even if it was, it will never be a speed demon with the 40hp.

Have you check it?

Has it always ran at this speed?

Have you just bought the boat?

 

[DutchMerc]

Re: Power and speed

Have you check it?

Has it always ran at this speed?

Have you just bought the boat?

I just bought the boot, and the motor has been a sleep for over 12 years, they said. I replaced the impellor, cleaned the carb and replaced some gaskets and both sparkplugs. When cranked both plugs did spark.
Just to be sure I will check the two cilinders again...

 

[DutchMerc]

Re: Power and speed

Yes, I'm certain that your engine is not running properly. If it's only giving you 5mph. Even too long a shaft would do better than that...much better. It sounds like your engine is running on one cylinder or you have a slipping prop hub.

I think i am on the problem now. I bought a scope and hooked i up to both plugs. Seems that one of them is missing some sparks. When the motor is running idle, the flash is not always triggered. Althoug the motor keeps running, i can hear (and see on my scope) that the 1st cilinder is not firing propperly. Maybe three sparks every 5 seconds??? Looking for the cause now.

Can anyone tell me how to set the timing? I can see the mark on the flywheel, but there seems to be no mark on the fixed part to align with...

 

[Benny1963]

Re: Power and speed

looks like in the picture you cav plate is low should be level with bottom of transom or a little higher if ya can keep it gripping

 

[DutchMerc]

Re: Power and speed

I think i will change my transom. I can gain about 4 centimers in height and still keep grip on the motor.
Maybe this will help, but it still does not solve my problem with my missing cilinder... I have to find the problem here. I don't know if it's the switchbox or maybe the stator. Does anyone know a reliably test to make sure it's the switchbox? It is an expensive part, so i need to be sure...

 

[Chinewalker]

Re: Power and speed

Hey Snapping Turtle! Cool pic in your tagline. Love the sound of a Quincy Looper!
- Scott

 

[SnappingTurtle]

Re: Power and speed

Hey Snapping Turtle! Cool pic in your tagline. Love the sound of a Quincy Looper!
- Scott

You and me both. Wish I could hear it behind me on my little boat!

 

[mthieme]

Re: Power and speed

I have a 35 Merc (same as 40) on the back of a 14' McKee Craft (similiar to Boston Whaler) and I do about 28 mph +/-. I haven't GPSed her though.

 

[DutchMerc]

Re: Power and speed

I fixed the problem with the switchbox; I replaced it and now both clinders are firing. Took the boat out yesterday, and got it to at a speed of 14Mph on the GPS. Not a verry good speed i think, but it gave a very big wave behind the boat. I think i have a weight-problem now, as the transom is verry deep in the water. I have to find the problem here....