Propping a SeaArk 1872 Pro CC.

I recently ordered a custom built SeaArk 1872CC with a Suzuki DF90A. Upon delivery the dealer lake tested the boat and said he was able to get 5800 RPMs out of it. I picked it up and returned home only to find I could barely get 5000 RPMs and 35 MPH out of it. The boat porpoises horribly too if you try to add even the slightest bit of trim. The numbers above were with a factory Suzuki 14X20 stainless prop. At this point I had to make a few assumptions; The dealer misread the tach and was only turning 4800, something had happened to the motor between the time he tested it and my testing the next day, or the prop changed pitch on the drive home. I went with the assumption that he had read the tach wrong since this would parallel more closely with my testing. The results would tend to indicate an over propping situation. The dealer swapped the 20p for a factory Suzuki 14X18 and things improved somewhat. I was able to get 5600 @ WOT. A 600 RPM jump from a 2" pitch change surprised me a little. Top speed went to 37 MPH. The proposing was not as severe but was still there if the trim was even touched from the fully down position. I'm still just barely into the operating range of 5500-6300 RPM. I've got a factory Suzuki 14X16 inbound (should be here today). If I see the same 600 RPM increase going from 18" to 16" that I saw going from 20" to 18" that should put me near the upper end of the operating range. I'm also starting to question the engine mounting height and will check that next time on the water. All that being said my questions are:

Does a 14X16 seem like too low of a pitch to run on a boat like this?
Should I be looking for a motor problem since the dealer claimed 5800?
Should the porpoising improve to the same degree I experienced going from the 20 to the 18?

1) Performance issue you are trying to correct. Porpoising and low RPMs
2) Current prop manufacturer, model, aluminum or stainless as a minimum. Suzuki Stainless Steel
3) Current prop diameter and pitch (required). 14X18
4) Wide open throttle RPM and speed with an average load (very helpful) 5600 RPM @~ 37 MPH
5) Engine/drive make, model, year, and HP Suzuki DF90A 2010
6) Boat make model, year, length and weight 2011 SeaArk 1872 Pro CC, 18', ~1000 lbs.

Re: Propping a SeaArk 1872 Pro CC.

I'm no expert but I have an 1860G3 jon that had similar problems because the motor was mounted too low. At all down the front of the cavitation plate was barely below the keel, and the CT plate is pointing up from parallel w the hull. any triming turns the CT plate further into the water passing under the boat and I believe causes porposing and alot of drag, slowing the boat. I raised my motor to # 2, up from #1 and pref. improved as well as range of tilt before porposing. I added a 4 blade alum prop. and it made a huge difference in overall perf. on my boat. If your boat is "custom" it may weigh more than the stock specs w added features. A friend has a 2072' Seark Rivercat w Suzuki DF90 2010, And it will run about 40, it has almost all options, and a hydraullic jackplate so his results may vary. Suzuki has "boat tests" On their website for the 18xv Seark boats that are evedently 60" and rated for 90hp, 45mph w 19"pitch I think?. The 1872 pro is rated for 130HP but not listed in the tests. Again I am no expert, just trying to help.

Re: Propping a SeaArk 1872 Pro CC.

Mine is an Express 1870 with a 90HP Yamaha, and it'll run wide open, trimmed up, to about 5400RPM, but at higher speeds just a little trim causes that porpoising, so usually just leave it trimmed down. My boat has a 6 degree deadrise at the transom, so it's mostly flat, and I believe these flat-bottomed jon boats just don't do very well trimmed up. Of course some trim tabs would probably help with the porpoising but we don't run much high speed since my 90HP 2 stroke tends to suck the tank dry real fast at higher RPM's. Hopefully you can make some adjustments to give you the performance you want and need! Good Luck!