Decreasing RPMs and MPH

[WildCatFan54]

Since I'm still in my rookie season of jet boating , I wanted to run something by the group of those more experienced than myself. My AR240 was launched on 4-30-16. It has been used almost every weekend since then but only 30 hours on the twin 1.8s as of last weekend. On 4-30 , after doing my break in at under 5000 RPMs for about 1 hour , we ran the boat at WOT with just my wife and I on board and a full (50 gallon ) fuel load. The boat rapidly accelerated to over 50 mph and about 7500 RPMs on both engines. Life was really , really good ! I did the 10 hour initial maintenance on the water , in my covered wet slip , on Lake Cumberland , Ky. I had purchased another set of (8) NGK plugs identical to what came in the boat from factory. I gapped them myself and applied a conservative amount of anti-seize. I kept the original set of plugs and was going to put them back in next season. I also replaced both oil filters after draining approx. 3 1/2 quarts of oil from each engine and replacing with a high quality synthetic blend of 10w-40. Both main bearings were packed with marine grease at that time. Most of what the family and I do on the lake is at very low speeds and RPMs . It takes us about 20-30 minutes just to get out of the "no wake" zone in the massive marina where we dock. Then we go less than a mile , drop anchor , swim and enjoy the day. My point is that since April 30 , until last weekend , I had never ask the boat to run at WOT. Last Sunday night the lake was perfectly calm , only about 20 gallons of fuel in the tank , I was alone and had a "need for speed". I was hoping for more than I got. It could only get to 40 MPH and the digital tach was bouncing between 6900 and 7000 RPM on the starboard engine. The port engine never touched 7000. So now I'm wondering what has changed ? Granted the boat has been in the water for about 5 months now and the hull I'm sure is covered in lake scum. Also , we keep the bimini up all the time. I know your not supposed to run the boat with bimini up over 40 MPH but I had a lapse in memory. Even though I tried , we did not run the boat , with bimini up , over 40 MPH !! It would seem to me that these factors would only affect speed ?? Shouldn't the RPMs still be higher even if the boat has slowed down ? I'm going down this weekend and take the boat out after putting the bimini down. I'm thinking about putting the original set of spark plugs back in both engines. The hull will not get cleaned until the boat is taken out of the water , towed home , winterized , and stored for the off-season. This will hopefully be early November unless I have more serious issues ? I check the clean outs after each use and have only found debris a couple times which is easily removed. Can anyone think of something I may be overlooking ? Thanks in advance for your time and thoughts.

 

[McMark]

Clean the hull and try it again on a day that was as cool as April 30. Check your air filters for oil. You should run your your boat at WOT at least once when you go out even during the break in period to seat the rings. If the rings didn't seat the engine can make oil (actually fuel dilution) and cause an overfill condition thus fowling the filters. If you had to gap the plugs, they are the wrong plugs. You shouldn't be gapping them. It stresses the metal and could fail/fall into the cylinder.

 

[McMark]

On a cool day my boat is a 50 mph boat every time. At 95 degrees it's more like 47 and 7300rpm.

 

[WildCatFan54]

Good call and thanks for pointing out the air filters ! I have not had them out for inspection since putting the boat in service. I should not have said that I gapped the plugs. It would have been more accurate to say that I checked to confirm that the gap was correct. It was.

 

[jobu2k]

Hey @WildCatFan54 . If you haven't read through my post from the past few weeks, you might give it a shot. My boat isn't new, so it may very well be that the situations aren't the same, but oil in the filter was definitely my issue.

 

[swatski]

Since I'm still in my rookie season of jet boating , I wanted to run something by the group of those more experienced than myself. My AR240 was launched on 4-30-16. It has been used almost every weekend since then but only 30 hours on the twin 1.8s as of last weekend. On 4-30 , after doing my break in at under 5000 RPMs for about 1 hour , we ran the boat at WOT with just my wife and I on board and a full (50 gallon ) fuel load. The boat rapidly accelerated to over 50 mph and about 7500 RPMs on both engines. Life was really , really good ! I did the 10 hour initial maintenance on the water , in my covered wet slip , on Lake Cumberland , Ky. I had purchased another set of (8) NGK plugs identical to what came in the boat from factory. I gapped them myself and applied a conservative amount of anti-seize. I kept the original set of plugs and was going to put them back in next season. I also replaced both oil filters after draining approx. 3 1/2 quarts of oil from each engine and replacing with a high quality synthetic blend of 10w-40. Both main bearings were packed with marine grease at that time. Most of what the family and I do on the lake is at very low speeds and RPMs . It takes us about 20-30 minutes just to get out of the "no wake" zone in the massive marina where we dock. Then we go less than a mile , drop anchor , swim and enjoy the day. My point is that since April 30 , until last weekend , I had never ask the boat to run at WOT. Last Sunday night the lake was perfectly calm , only about 20 gallons of fuel in the tank , I was alone and had a "need for speed". I was hoping for more than I got. It could only get to 40 MPH and the digital tach was bouncing between 6900 and 7000 RPM on the starboard engine. The port engine never touched 7000. So now I'm wondering what has changed ? Granted the boat has been in the water for about 5 months now and the hull I'm sure is covered in lake scum. Also , we keep the bimini up all the time. I know your not supposed to run the boat with bimini up over 40 MPH but I had a lapse in memory. Even though I tried , we did not run the boat , with bimini up , over 40 MPH !! It would seem to me that these factors would only affect speed ?? Shouldn't the RPMs still be higher even if the boat has slowed down ? I'm going down this weekend and take the boat out after putting the bimini down. I'm thinking about putting the original set of spark plugs back in both engines. The hull will not get cleaned until the boat is taken out of the water , towed home , winterized , and stored for the off-season. This will hopefully be early November unless I have more serious issues ? I check the clean outs after each use and have only found debris a couple times which is easily removed. Can anyone think of something I may be overlooking ? Thanks in advance for your time and thoughts.

Air filters could be an issue, fairly easy to test, as @jobu2k did.

But -- more than likely it is the dirty hull that slows you down. Clean it with a good hull cleaner (w/oxalic acid) and wax. There are many opinions on what wax to use, but the bottom line is - you want a clean shiny hull. You will be back in high 40s and hitting 50 in no time. If you have any scum/growth on the hull, don't even bother entertaining any other reasons for your speed loss. You can loose 30% of top speed (or more) over few weeks of wet slipping.

 

[WildCatFan54]

Thanks jobu2k and swatski. Cleaning and waxing the hull is probably not going to be an option for me until mid November but I won't worry about it too much over the winter knowing that it will be faster next spring. I will be taking the air filters out for inspection this weekend.

 

[Bruce]

If your boat has been sitting in the water for months then your hull is slowing you down.

 

[Nkc240]

Having the same problem with my 15 ar240 that I got in April, brand new. About 60 hours in now and has never left the water since April. I recall doing max speeds of about 52-54 mph when I first got it. Did an oil and filter change at 20 hours.

Went out for a rip today and only got up to 44mph max at 7500rpm on both engines. Still scratching my head on this one. Hopefully it's just a scummy hull slowing me down or maybe the wife gained weight.

Also, anyone ever have a faulty cruise control? My port engine light keeps going on and we all know what that sounds like. Just annoying to constantly having to press the mute button on the screen. The cruise control works about half the time. Any fix for it?

Thanks.

 

[WildCatFan54]

Having the same problem with my 15 ar240 that I got in April, brand new. About 60 hours in now and has never left the water since April. I recall doing max speeds of about 52-54 mph when I first got it. Did an oil and filter change at 20 hours.

Went out for a rip today and only got up to 44mph max at 7500rpm on both engines. Still scratching my head on this one. Hopefully it's just a scummy hull slowing me down or maybe the wife gained weight.

Also, anyone ever have a faulty cruise control? My port engine light keeps going on and we all know what that sounds like. Just annoying to constantly having to press the mute button on the screen. The cruise control works about half the time. Any fix for it?

Thanks.

Nkc240 , I'm not going to "weigh" in on your wife's situation but you mention something that I questioned in my original post. It seems that you are still getting close to max RPMs at 7500 ? Your speed has decreased to 44 MPH which I would kill for at this point. My RPMs have dropped in the 6900-7000 range to go along with slowing down? It seems to me that I should still be up around 7500 RPMs unless I also have some oil accumulation in my air filters plus my scummy hull ?

 

[Nkc240]

Nkc240 , I'm not going to "weigh" in on your wife's situation but you mention something that I questioned in my original post. It seems that you are still getting close to max RPMs at 7500 ? Your speed has decreased to 44 MPH which I would kill for at this point. My RPMs have dropped in the 6900-7000 range to go along with slowing down? It seems to me that I should still be up around 7500 RPMs unless I also have some oil accumulation in my air filters plus my scummy hull ?

It wouldn't be hard to check out the air filters just to be sure. It only takes about 5 mins. I would also check your throttle cable to see if its going all the way.