Boat speed question

[Will191]

1) Performance issue you are trying to correct.
I'm new to boating having bought this boat two months ago. What I'm trying to correct is that I'm unable to get boat to plane. Both motors seem to run strong, good oil pressure, but it just seems to churn water and won't get going. Trim tabs (Bennet) function but it still won't plane. There is some buildup of gunk on the bottom of boat where it sat in fresh water for several years prior to purchase, and I'm planning on having the bottom painted, but haven't yet. I'm not sure how much of a factor this is and I haven't been under the boat to see how much is there. From the sides it looks minimal, but I don't know enough about boats to really judge. One thing I'd like to know is what should I expect speedwise from a boat like this. I've experimented with the trim tabs and pitch, but it just seems like all she wants to do is plow.

2) Current prop manufacturer, model, aluminum or stainless as a minimum.
Solas 4-blade aluminum. Some discoloration on prop, some pitting around the edges, but generally appears to be in good shape. No notches or dents that I can find.

3) Current prop diameter and pitch (required).
14.5 X 17

4) Wide open throttle RPM and speed with an average load (very helpful)
4000rpm yields 14mph. Load: two adults, two children. approx: 580lbs. Boat has Cruisair air conditioner, Onan 3.0 generator, fresh water (35gallons, I think) 100gal of fuel. I guess that's my average load.

5) Engine/drive make, model, year, and HP
Twin 3.0 Mercruisers, 1986, 140hp each (181cid), drive I/O.

6) Boat make model, year, length and weight
1986 Chris-Craft Scorpion 266. I've been unable to locate an authentic owner's manual for this boat, but what I've found on iboats says the boat should weigh 5200lbs dry. I'm not sure if this is for two motors and all the accessories, like the generator and a/c, so maybe 6000lbs. All the boats I've located on the internet, approx 4, all have a single V-8 Merc 260, and don't mention accessories but do reaffirm the 5200lbs. Draft is approx 2 to 2.5 feet. Beam is 8.5ft.

Any help/advice would be greatly appreciated.

 

[Bondo]

Re: Boat speed question

There is some buildup of gunk on the bottom of boat where it sat in fresh water for several years prior to purchase, and I'm planning on having the bottom painted, but haven't yet. I'm not sure how much of a factor this is and I haven't been under the boat to see how much is there. From the sides it looks minimal, but I don't know enough about boats to really judge.

Ayuh,... Welcome Aboard,...

Clean the boat's bottom, 'n try it again,...
I'm bettin' that's yer Problem....

 

[steelespike]

Re: Boat speed question

I agree a clean bottom would be a good start. Your twins at a total 280 hp should do well.
probably not quite as fast as a single but should do well.
In the mean time be sure the motors are in excellent tune, timeing right and advancing as it should. Be sure the throttles open all the way.Be sure the trim is going all the way down.
Do you trim down for hole shot?

 

[CaptainKickback]

Re: Boat speed question

You have 280 HP. I had a 23 Walkaround with a single 225 OB. Wit several people sitting in the back, it took forever to get on plane. What I'm saying is the your boat is minimally powered to begin with, so the weight and the friction from the bottom build are probably having significant effect. Also, the stuff on the bottom could cause cavitation (air in the water) and your props have nothing to push against.

Also check to make sure the hubs in your props are in good shape. If bad, they could drive the boat slowly but slip under more stress.

 

[CaptainKickback]

Re: Boat speed question

Your twins at a total 280 hp should do well.
probably not quite as fast as a single but should do well.

Steelespike, I'm curious. Why do you say a single would be faster than twins?

 

[Will191]

Re: Boat speed question

Thank you for the replies.

Ayuh,... Welcome Aboard,...

Clean the boat's bottom, 'n try it again,...
I'm bettin' that's yer Problem....

I'm having the boat pulled out for bottom painting tommorrow morning. I'll report back afterwards.

I agree a clean bottom would be a good start. Your twins at a total 280 hp should do well.
probably not quite as fast as a single but should do well.
In the mean time be sure the motors are in excellent tune, timeing right and advancing as it should. Be sure the throttles open all the way.Be sure the trim is going all the way down.
Do you trim down for hole shot?

These motors have points and condensers. Haven't fooled with those since my Jeep days, but they look simple enough. I didn't notice a vacuum advance on these 3.0s. What advances the time? I've checked the trim tabs and they seem to be going all the way down. I have not trimmed them down for a hole shot, but I certainly will try it after the bottom gets painted and let you know the results.

You have 280 HP. I had a 23 Walkaround with a single 225 OB. Wit several people sitting in the back, it took forever to get on plane. What I'm saying is the your boat is minimally powered to begin with, so the weight and the friction from the bottom build are probably having significant effect. Also, the stuff on the bottom could cause cavitation (air in the water) and your props have nothing to push against.

Also check to make sure the hubs in your props are in good shape. If bad, they could drive the boat slowly but slip under more stress.

Is there a way to check the hubs without removing the props?

 

[steelespike]

Re: Boat speed question

Steelespike, I'm curious. Why do you say a single would be faster than twins?

The twins are likely heavier and your dragging two lowerunits through the water.

 

[steelespike]

Re: Boat speed question

While I've never worked in an I/O except to change plugs I believe they have centrifigal
advance and they have been known to get stuck.

 

[Will191]

Re: Boat speed question

Ayuh,... Welcome Aboard,...

Clean the boat's bottom, 'n try it again,...
I'm bettin' that's yer Problem....

I had the boat's bottom painted and was able to get her to plane. I was able to reach 27mph at 4800rpm which I believe is WOT. The boat was still accelerating slowly when I approached a group of sailboats, so I had to slow down. I'll try her again this weekend. Does 27mph sound about right for this boat?

One side question though. I had the sterndrives all the way up, just prior to painting the bottom, to inspect the props and forgot to put them down when the mechanic came to paint the boat. He mentioned this, but I saw the boat out of the water in the paint shop and the drives looked down. The boat was in the slip when I got it back to try it out and I forget to see if the drives were all the way down. After I got her to plane I remembered this and was attempting to lower the port motor all the way down when I ran into a problem. I have no indicator to tell me if the motors are down so I was idling back out of the marina and listening to the port motor actuator whine, waiting for it so bottom out, which always resulted in the higher pitched whine lowering suddenly, but instead of bottoming out, it just quit. I could hear a clicking noise where the switch was trying to engage, but there was no whine either to lower or raise. Just a click. I guess the motor was all the way down and I didn't know it and screwed something up. Is there a fuse I should look for or did I do something serious? The starboard actuator still functions normally.