54' Offshore won't get on plane

Agree you need 2330 if she were my boat. We surveyed a 54 with C12's out of FLL 2 months ago and she was advertised to cruise at 16 knots. 19/20 wot. At 1950 we had 12/12.5. WOT was only 2220
When we stripped off the prop speed we found the props had been damaged and repaired multiple times. New set of Veems 3 weeks away. The new props are set to give us 2320/2340 wot.
These Offshore 54' struggle to get up and trim off anyway so everything has to be 105%.

The problem has to be something mechanical. It happened over a short period of time. The Cats are turning up to rpm. CSkipR said in post #17 it was shuddering when about on plane, that sounds like prop cavitation. The underwater exhaust is getting to a prop or props. Could be a combination of a trim tab and the stabilizer fins. Had this very same problem sea trialing a Marlow. The STB trim tab was stuck in the full down position and caused the stabilizer to constantly compensate which was washing the exhaust into the STB prop and cavitating. The boat would not come up to speed or plane. Have someone check the engines when shuddering, the engine cavitating should noticeably move a lot. You could also manually lock the stabilizer fins in the center position with the stabilizers off to check if it resolves the problem.

NYCAP I agree with your assessment. The OP says it happened all at once, not slowly over time. If that assessment is true, and not missed perceptions of a creeping problem, the last thing I would do right now is change my bottom. Based on the fact pattern given us, IMO some "event" happened. Maybe it was the extra two beer cases that were loaded on, if the ride was that marginal, maybe its malfunctioning running gear as discussed above?

I do think , though, that no one here is recommending that their proposed solution be instituted until the true problem is discovered.

If the engines are making 2200 rpm, then by definition they are making enough power. I would verify that the gears are not slipping.

They should be 2300 rpms WOT. So how are they making enough power if they only hit 2200 rpms?

Almost all the mechanicals have been checked, including the props, although I would recheck the tabs since that's easy and because they're so critical for getting this boat on plane. However my money is still on water intrusion either in that cavity he mentioned or the hull soaking it in. With a boat that barely gets on plane the growing problem wouldn't be noticed until one day it didn't. The shuddering could also be caused by the mechanicals working fine and trying to get the boat to get to where it'll no longer go. The boat hasn't had an extensive dry dock (dry out time) in at least 3 or 4 years. I'd haul, check that cavity and put a moisture meter against the hull. Then if I got the results I expect I'd put her on the hard for the next 6 months or so, and while it's up I'd put on bigger tabs. Having the tabs full down to keep on plane also increases drag. Bigger tabs would give him more lift while reducing drag.

Read above. They made 2200 rpms and they didn't try for 2300. More importantly, the boat used to plane at 2200 rpms. The thing that many people don't understand is that engines turn props and props move boats. If the props are turning, the engines are doing their job. If the boat isn't moving, look elsewhere (starting under water).

J you continue to blow smoke on this topic and cloud the OP issue with your nonsense. It seems very typical of your weekend late night posts and has driven away many technical members on this forum, you truly earn your bully pulpit status.

Maybe that is your end goal anyways, if it doesn’t happen in your backyard or in your vision it doesn’t seem to exist until you do your frantic google searches and reply with more nonsense. I am sure all the yards in California are incompetent and “backyard boat yards” because you just say so. Amazing perspective from a ‘Captain”.

I have run on the 48 Offshore Sedan, which does really well out West from Alaska to Mexico. I have been in the California boatyards since the 80’s and have seen countless Offshore yachts next to our personal boats and those I have helped manufactured, in and out of the water.

Enjoy your South Florida bubble.

715hp C12's are 2300 rpm motors, you don't cruise them there. I don't think they make nearly 2300 rpms. In a heavy boat with borderline power, It's critical in a situation like this to make over 2300 rpms, so that your cruise rpm is 100 or so rpm's higher (where it should be) and therefore your turbo boost is also higher, and you're making more torque and hp, and the props are spinning faster to create more lift to carry the heavy load. I've seen it before on another boat that was underpowered. An Arneson boat that if the props had more than 6 barnacles, it wouldn't get on plane.

I don't dispute the importance of making full rpms but for all either of us knows this boat does make full rpms (did you read OP's comment on this issue?). As to the issue at hand -- resolving the boat's failure to plane at 2200 rpms -- it cannot be a lack of power problem as you asserted. Let's try to be helpful to the OP, and not lead him off on a tangent.

The boat all of a sudden has an issue, when it used to do 14 knots 2 months ago. This points to something being wrong with either the engines, stabilizers, weight or something else. SO the logical step would be to fix the problem.

What owner would all of a sudden start adding fiberglass to the stern to create hook and modifying the hull bottom of the boat???? It's crazy to even recommend modifying a hull in this situation and could really hurt resale in the future and/or have negative handling effects. AND, you still haven't fixed the origional problem!!!!!!!!!!!

"A friend has owned this boat for approx 3-4 yrs. The boat has C18 Cats 700hp ea and about two months ago it would not get up on plane. Its a relatively slow boat and when on plane about 14 knts. Bob from Cat came over about a month ago and said its not an engine problem it was likely props. Thats when owner had boat pulled and all the bottom work done. The bottom was painted, new cutlass bearings, props pulled & scanned by General Prop, rudder and trim tabs both check okay. Put back in water and still same problem. The engines will get up to 2200+rpm and boat will be struggling trying to get on plane. Any ideas on what the issue could be."

CAT mechanic also recommended it was in the props. He says that engines will get up to 2200+ rpms and boat struggling to get on plane. It sounds over propped.

If nothing was added and no major weight shifted in the boat. I'd verify the trim tabs have full travel. Make sure the water tank is full if it's in the bow. Bottom clean. Pin the stabilizers in the centered position and then seatrial. Verify which side the major vibration is from, or both sides. At least if the vibration is only coming from one side, you can narrow down the issue to that side. The vibration is also an issue that's relatively new and probably coincides with this issue. Could be a bent shaft and that is effecting performance and causing the performance issue. Props were scanned but is it possible they're flexing? Motor mount issues? A gear that's coming apart?

Keep reading if you think the full rpm question is important, as OP clarified that concern for another poster. Better yet, focus on the facts we are given (the boat used to plane at 2200 rpms but no longer does, even though it still makes 2200 rpms now) and explain how more power, at 2200 rpms could change anything, let alone the failure to plane. Maybe I will learn something from you, but I don't think it is possible, and if I am right, the problem is other than a lack of power.

I see that post where he mentions, it makes 2300rpm..... 2200 rpms + is a broad number. But if you're turning the props the same exact rpm's, the boat should do the same speed (unless there's something causing more prop slip). Anyways, what I did pick up on is post 17. NOW, the boat starts shuddering really badly and didn't do it before the issue.

Since this just started 2 months ago and the owner has owned the boat for 3-4 years, I doubt it is the placement of the underwater exhaust. Some things that come to mind. 1 of the stabilizer fins shifted on the shaft or the shaft got bent, and is now causing drag and the boat to crab and directing exhaust gasses into a prop. A bent rudder causing the boat to crab (I think this one if highly unlikely) or a shaft that's about to become two piece and whipping and causing the prop to move around. Need to look at the engines and see if one or both are shuddering to determine if it's one side or both sides.

They worked hard to earn the nickname "Taiwanese Tub" and it's typical performance (or lack thereof) for them. How about the 66' MY that does ohhh 17 knots WOT with 2000 hp?

CskipR- that's good that it just turned out to be a wire on the stabilizer and not something more serious. Unfortunately it took this much $$$ to figure it out.

Sadly, that boat is the exception and not the norm for Taiwanese boats. One of the few Taiwanese MY's that performs well. Look at the 85' Johnson for comparison, 14 knot cruise..... whereas the 85' stretched to the 105' (same hull) actually cruises at 21 knots........