MK-IV Long Beach





Length: 68'
Beam: 18'
Draft: 4'
3 - Detroit Diesel 12V71TI, 2  engines unruffled, 1 muffled
Generators: 2 - Onan, 15KW Gensets
Speed: 25+ knots
Crew: 5+1 
2 - M242, 25mm chain gun
1- Mk 2, 81mm Mortar w 1-M2 HB .50 cal MG
2 - M2 HB .50 cal MG on Multi-purpose mount
2 - Mk 19 grenade launchers on Multi-purpose mounts
Small arms:  Various



MK IV 68’ Patrol Boat by Howard Nash, QM1(SW/CC), USN (Ret) (02-14-09)  


The MK IV 68’ Patrol Boat was ordered by Commander, Naval Forces, Southern Command (NAVSO) in 1984. Three boats were ordered to increase the capabilities of Harbor Patrol Unit, Combat Craft Division, Panama Canal in the defense of the Canal and in other tasking set forth by Commander-in-Chief, Southern Command (SOUTHCOM). 


The MKIV’s were based on the MKIII 65’ Patrol Boats built by the Peterson Boat Building Co. in Wisconsin and use by Special Boat Units since 1973. The MKIII’s themselves, were based on oil rig crew boats in the Gulf of Mexico, as were the MKI Patrol Boats and MKI and MKII PCF’s (Patrol Craft, Fast) or “Swift” boats.

The MKIV’s were extended three feet, primarily to accommodate three, Detroit Diesel 12V71TI engines. The MKIII had three 8V71TI engines. The upgrade was required to accommodate the weight of the 2 M242 Bushmaster, 25mm Chain guns, which was designed as the primary weapon o the M3 Bradley Fighting Vehicle,  and the general consensus was that the MKIII’s were underpowered, especially when conducting  RDT&E on systems such as the Penguin Missile.           


Two boats, 68PB842 and 68PB842,  arrived at Naval Station Rodman, Panama, in December, 1985. 68PB843 arrived three weeks later. All boats were shipped via sealift as were the weapons. The installation of the two chain guns, and 81mm mortar were installed within a week.

Prior to installation of the weapons, the top speed with full fuel tanks was 29knots. With weapons installed and with a full load, the speed was recorded at 26.7 knots.

Unlike the MKIII’s, which had unpainted upper hulls and superstructures, the MKIV’s originally were painted Deck Gray. Much debating took place on this issue. The unpainted hulls were said to absorb radar, therefore, had a weak radar signature. Some said they should be Haze Gray, although leaving the boats unpainted would have the same effect, color wise. Black was discussed briefly, as the PCF’s and PBR’s in the unit were of this color so, the compromise of deck gray was done. It worked well at sea and sufficiently in the jungle background of Latin America. Although a PCF, at 50 ft could hide reasonably well on the banks, a PB at 68 ft could not.


The first deployment for the MKIV’s was to Puerto Cortes, Honduras. Executed by PB841, with GMCM(SW/DV) Ray Stewart as Boat Captain/Patrol Officer and PB842, that was skippered by BMCS(SW) Bob Clayton. The OIC was LCDR Jeff Lucas. Deep freezers were placed in the lazarettes and 8 55gal drums of DFM were lashed to the port and starboard sides of the fantail.

After transiting through the Canal and overnight at Shelter Cove, Ft. Sherman, the MKIV’s left the Limon Bay breakwater into the Caribbean Sea on the first leg of the journey. The sea state was such that as soon as the breakwater was behind only the Boat Captains and Navigators, QM1 Howie Nash  on 841 and QM2 Mark Nesbitt  were the only men not overcome by sea sickness.

Three days later, the boats called at Isla San Andreas for fuel. The island is approximately 5 miles long and 2 miles wide. It is Colombian soil located about 60nm from the east coast of Nicaragua. Actual arrival was the evening before. It was deemed however, to anchor on the leeward side of the island until daylight. None of use had ever been to San Andres previously. The sick arose from the dead and the barbeque commenced that night. The boys were hungry after yakking nothing but air.

The next morning, we entered the harbor, moored pier side and took on fuel and freshwater that our Corpsman, HM2 Diego Gonzales highly treated. Diego had the watch as he could not leave the boats, which under Free Practique, was sovereign, US soil. See Diego was of Colombian birth and we were afraid he would get shanghaied (held for military service) by the Colombian authorities since he was still a Colombian citizen. I myself hit the rack as I had been up the entire 3 days.

The next morning was the beginning of the final leg to Honduras. As soon as we left the  comfort of the harbor, Boom! Down went the boys again. Mark Nesbitt and I took turns navigating the detachment so we could get some kind of sleep. 3 hours a day was about it.

After turning west at the corner where Nicaragua and Honduras form a border, the seas turned to glass.  The animals rose from their caves and refueling from the drums of diesel began. That evening we passed between mainland Honduras and the Bay Islands.

We learned something upon arrival in Puerto Cortes that mid morning. The deck gray paint did serve a purpose. It scared the crap out of people. Good choice after all, I’d say.

After a bit of training with the Hondos for two weeks we headed back to Panama under more favorable seas. LCDR Lucas even split the navigation duties with Mark and myself.


The primary benefits that came out of the MKIV’s were the US Navy had a long-range asset that it could deploy at a relatively low cost to show the flag in the AO, and a heavy weapons platform that would prove itself later, in Operation Just Cause. Primarily at the hands of BMC Jerome Little.

The unsung benefit was the success of the M42 Bushmaster Chain Gun. Previously tested by SPECBOATRON 2 in Little Creek, VA, it was seen as unreliable due to exposure of the electrical components to saltwater. The first crews of the MKIV’s, proved this incorrect. So much so, that the US Navy began installing them on ships during Operation Earnest Will in the Persian Gulf and was the main battery for the PC’s, now in the hands of the US Coast Guard.


US Navy Development  of the M242, 25mm Chain Gun “Bushmaster”

The Bushmaster was originally developed for the US Army M3 Bradley Fighting Vehicle. It was seen as a possible replacement for the 20mm cannon used as the main battery on MKIII Patrol Boats and as ship self defense weapons against small boat attack.

The Bushmaster was tested at Special Boat Squadron Two, in Little Creek, VA in the 1982-83 timeframe and found unsatisfactory for its apparently high maintenance due to the saltwater environment.

The project was shelved by Special Warfare, where upon, Harbor Patrol Unit/Combat Craft Division, Panama Canal, ordered three 68’ MKIV Patrol Boats. The main armament was chosen to be the Bushmaster. The weapon was allowed to be tested one more time. The first tests were performed nearly exclusively by one GMG1 that, as it turned out, had a personal dislike for the weapon.

After successful testing of the experiencing the firepower of the weapon and it’s different forms of ammunition and the success of the maintenance program, the Bushmaster was retained aboard the MKIV PB’s. In addition, the success was so overwhelming that the weapon was placed aboard surface ships in the Persian Gulf to suppress any small boat attacks.

The Bushmaster may have been cast aside for use on the MKIII PB’s because the power requirements would make it necessary to install an additional 15KW generator. 


The Bushmaster is a battery powered, gear -driven, chain operated, semi and automatic  weapon. It fired 25mm practice, ball, and depleted uranium (DU) rounds. No blank firing capabilities existed.

The ammunition belts were boxed in 150 round belts that required 2 men to load into the box magazine on the port side of the weapon. While one man cranked the hand-wheel to feed the chute that led to the breach, the other man guided the belt into the box. This and bore sighting the weapon were the only evolutions that required more than one person to execute.

The operator had to be of certain height and build to control the weapon with accuracy as the original intent of the developers was for the Bushmaster to be a turreted gun. The gunner placed his shoulders into the horseshoe -shaped, foam covered brackets, and strapped in with one web strap. The sighting was done through an Aimpoint sighting system. Two handles were provided with the right being an electrically operated trigger. The system was battery powered and recharged by a 15kw generator.  When depressed, the trigger set into motion the motor that drove internal gear that in turn, drove an extremely large chain  to activate the firing sequence. The recommended rate of fire was three-round bursts. Full automatic would whip a man around due to the recoil, much less greatly decrease its accuracy. Still, it was an extremely accurate and powerful weapon when used properly. The gun mount was well balanced and could be manipulated easily both when traversing and elevating.

(12-11-02) I believe the Mk IV PB is an improved Mk III PB. I found a really neat site that has a table of Navy small combatant craft including year of mfr, number built, serial numbers, etc. Do a Google search on grafton boats and then look for the entry that says "numbered patrol craft." From the table, there were 3 Mk IV's built by Peterson (builders of the Mk III PB) in 1985 and she was 68 feet long. I'd need to see additional shots of the boat for a better analysis. If this boat shows a Mk IV in Panama, then the boat was probably attached to SBU-26 (now decommissioned). SBU-26 had PBR's to defend the Panama Canal, but it would not surprise me to learn they got Mk IV's. I would bet the boat got turned over to the Panamanians when the SBU went out.

I just got an e-mail from Jim Gray on the Mk IV. He confirms that the Mk IV was a product improved Mk III with 12V71 diesels (vice 8V71), new electronics, and had the Mk 38 Mod 0 25mm "chain guns" instead of the usual Mk III PB armament. All three boats went to SBU-26 in Panama. Before SBU-26 was decommissioned, they go the pre-standardized paint jobs used on the Mk V SOC.