Trojan said:
I was told earlier that My 43 should WOT at 2150,cruise at 2000rpm. I just received the data sheet on My boat from Hatteras today. WOT 2500rpm, no load 2650rpm, cruise at 2200-2300. At WOT I show 20.5 mph.. At 2300rpm I show 17.5mph., 2200rpm I show 15.5. This are the speeds I get with 400 gal of fuel and the boat loaded. The boat came with 24X26 props. I am running two beat up props of 24X25 pitch. Which need to be changed. The question is? Do I leave well enough alone or do I change to 24X26? I don't know what I will gain if I change. What I have now with the warn props looks pretty good to me. Any thought on this. Bill
First things first.
Get the engines warmed up, and get ahold of a phototach - the bridge tachs, unless they are digital (and sometimes even then!), are worthless.
Your engines are probably 6-71s, right? I suspect so from the numbers you're posting.
Once you have them at operating temperature and that phototach, take the boat out of gear, go below, and BY HAND (disconnect the throttle linkage so you can move the lever yourself) go all the way to the stop. Measure the RPM. It should read between 2650 and 2700, if the no-load is 2650 (+50/-0).
Let's say it DOES NOT. You need to have someone come and fix that - first. Go no further until you do. I won't try to go through the procedure here, because it basically requires a full tune-up, which if botched can cause real bad things (like a runaway!)
Let's say it DOES. Ok. Now shut down and reconnect the linkage.
Make ABSOLUTELY CERTAIN that the lever goes ALL THE WAY TO THE STOP when you select WOT from the bridge. With the engine stopped, select full power and go down and make CERTAIN. If it does not, fix the linkage (this is adjustable at either the bridge controls, the linkage at the engine, or both.) Make sure of course that you can get idle too!
Ok, so now you know that when you say "WOT" at the bridge, you have WOT.
Now, dive the boat. Insure the bottom AND PROPS are clean. The props must be COMPLETELY clean - if they are not, get a scraper and some dive gear and clean them, or have someone do it for you.
Now, with the bottom and props clean, go for a speed trial.
If WOT is supposed to be 2500, you want to turn up
fully loaded 2550. This means with a clean bottom, clean props, and FULL fuel, water and provisions to the maximum extent you usually have when running. Why 2550? Because your props will not always be spotlessly clean, your bottom will not always be perfectly clean, and you will some day have an extra 500lbs of stuff (people, provisions, etc) on board. This will insure you do not overload the engines even under those conditions.
If you cannot do so, then you need to first have the engine tune verified, and if it checks out, then the wheels MUST be adjusted.
Take the wheels off and take them to a prop scanning place ("Accuprop" or similar.) Tell them what you have now for WOT, what you want for WOT (2550), and ask them to fix it. They will get very close if not dead nuts on - they will generally take 1" out for each 100 RPM you want to increase by, although you can also play with the cup to get some of these results. They will also true the wheels which helps both efficiency and vibration.
In general you should run Detroits at
no more than 87% of rated WOT RPM for cruise ASSUMING you make AT LEAST rated speed, and preferrably, 50 RPM over. That would be a maximum cruise RPM of 2175, assuming you can reach at least 2500. Operation over 2175 should be reserved for emergency situations.
If you exceed these numbers (e.g. the "2300 rpm" recommendation) or do not correct the overloading you are operating under right now you will be overhauling the engines much sooner than you'd like.
